61-80 of 182 Results

Article

Homeostatic Plasticity in the CNS  

Peter Wenner and Pernille Bülow

Homeostatic plasticity refers to a collection of mechanisms that function to homeostatically maintain some feature of neural function. The field began with the view that homeostatic plasticity exists predominantly for the maintenance of spike rate. However, it has become clear that multiple features undergo some form of homeostatic control, including network activity, burst rate, or synaptic strength. There are several different forms of homeostatic plasticity, which are typically triggered following perturbations in activity levels. Homeostatic intrinsic plasticity (HIP) appears to compensate for the perturbation with changes in membrane excitability (voltage-gated conductances); synaptic scaling is thought to be a multiplicative increase or decrease of synaptic strengths throughout the cell following an activity perturbation; presynaptic homeostatic plasticity is a change in probability of release following a perturbation to postsynaptic receptor activity. Each form of homeostatic plasticity can be different in terms of the mechanisms that are engaged, the feature that is homeostatically regulated, the trigger that initiates the compensation, and the signaling cascades that mediate these processes. Homeostatic plasticity is often described in development, but can extend into maturity and has been described in vitro and in vivo.

Article

Hormones and Animal Communication  

Eliot A. Brenowitz

Animals produce communication signals to attract mates and deter rivals during their breeding season. The coincidence in timing results from the modulation of signaling behavior and neural activity by sex steroid hormones associated with reproduction. Adrenal steroids can influence signaling for aggressive interactions outside the breeding season. Androgenic and estrogenic hormones act on brain circuits that regulate the motivation to produce and respond to signals, the motor production of signals, and the sensory perception of signals. Signal perception, in turn, can stimulate gonadal development.

Article

The Hypocretin Arousal Network  

Jeremy C. Borniger and Luis de Lecea

The hypocretins (also known as orexins) are selectively expressed in a subset of lateral hypothalamic neurons. Since the reports of their discovery in 1998, they have been intensely investigated in relation to their role in sleep/wake transitions, feeding, reward, drug abuse, and motivated behavior. This research has cemented their role as a subcortical relay optimized to tune arousal in response to various salient stimuli. This article reviews their discovery, physiological modulation, circuitry, and integrative functionality contributing to vigilance state transitions and stability. Specific emphasis is placed on humoral and neural inputs regulating hcrt neural function and new evidence for an autoimmune basis of the sleep disorder narcolepsy. Future directions for this field involve dissection of the heterogeneity of this neural population using single-cell transcriptomics, optogenetic, and chemogenetics, as well as monitoring population and single cell activity. Computational models of the hypocretin network, using the “flip-flop” or “integrator neuron” frameworks, provide a fundamental understanding of how this neural population influences brain-wide activity and behavior.

Article

Hypothalamic Control of Female Reproduction  

Brian P. Kenealy and Ei Terasawa

Female reproduction is an interplay between the hypothalamus, pituitary, and ovaries. While the gonadotropin releasing hormone (GnRH) neuron in the hypothalamus regulates gonadal function through the pituitary, GnRH neuronal activity is also profoundly influenced by ovarian steroid hormones. GnRH is released from GnRH neurons in a pulsatile manner after integration of a diverse array of internal and external milieus. Since the discovery of the mammalian GnRH molecule, over a dozen GnRH forms have been identified in the animal kingdom, and large numbers of publications in various lab animal and human studies suggest that GnRH neurons are regulated by multiple neuromodulators in the brain, such as kisspeptin, neurokinin B, β-dynorphin, neuropeptide Y, GnIH, GABA, glutamate, and glial factors. A recent emerging concept is that steroids synthesized locally in the hypothalamus, namely, neuroestradiol and neuroprogesterone, also contribute to the regulation of GnRH neuronal activity, and hence female reproduction. Together with modulation by various inputs and ovarian steroid feedback, GnRH neurons are responsible for puberty, cyclic ovulation, and menopause.

Article

Inflammatory Signals and Sexual Differentiation of the Brain  

Margaret M. McCarthy

Sex differences in the brain are established by the differential gonadal steroid hormonal milieu experienced by developing male and female fetuses and newborns. Androgen production by the testis starts in males prior to birth resulting in a brief developmental window during which the brain is exposed to high levels of steroid. Androgens and aromatized estrogens program the developing brain toward masculinized physiology and behavior that is then manifest in adulthood. In rodents, the perinatal programming of sex-specific adult mating behavior provides a model system for exploring the mechanistic origins of brain sex differences. Microglia are resident in the brain and provide innate immunity. Previously considered restricted to response to injury, these cells are now thought to be major contributors to the sculpting of developing neural circuits. This role extends to being an important component of the sexual differentiation process and has opened the door for exploration into myriad other aspects of neuroimmunity and inflammation as possible determinants of sex differences. In humans, males are at greater risk for more frequent and/or more severe neuropsychiatric and neurological disorders of development, many of which include prenatal inflammation as an additional risk factor. Emerging clinical and preclinical evidence suggests male brains experience a higher inflammatory tone early in development, and this may have its origins in the maternal immune system. Collectively, these disparate observations coalesce into a coherent picture in which brain development diverges in males versus females due to a combination of gonadal steroid action and neuroinflammation, and the latter increases the risk to males of developmental disorders.

Article

The Insect Central Complex  

Stanley Heinze

The central complex (CX) is the only unpaired brain region of the insect brain. It is located at the interface of sensory processing and motor control and plays a vital role in context dependent action selection. The CX has four main tasks. First, the encoding of the insect’s orientation in space, i.e., the generation of an internal head direction signal based on both rotational self-motion and external sensory signals. Second, the generation of goal direction representations. Third, the selection of an appropriate goal direction based on context, internal state, and previous experience. And finally, the initiation of motor steering signals based on comparing heading direction and goal directions. The highly regular, almost crystalline neuroarchitecture of repeating computational elements provide the structural basis for these computations. These tight structure function relationships have revealed that the CX performs highly efficient, vector-based computations, in which vectors are encoded as sinusoidal activity patterns across populations of neurons. The deep insight into the computational algorithms implemented in this brain area have made the CX a prime model system to study the neural basis of context-dependent action selection and behavioral decisions, as well as the mechanisms of circuit evolution.

Article

Insect Color Vision  

Natalie Hempel de Ibarra and Misha Vorobyev

Color plays an important role in insect life—many insects forage on colorful flowers and/or have colorful bodies. Accordingly, most insects have multiple spectral types of photoreceptors in their eyes, which gives them the capability to see colors. However, insects cannot perceive colors in the same way as human beings do because their eyes and brains differ substantially. An insect was the first nonhuman animal whose ability to discriminate colors has been demonstrated - in the beginning of the 20th century, von Frisch showed that the honeybee, Apis mellifera, can discriminate blue from any shade of gray. This method, called the gray-card experiment, is an accepted “gold standard” for the proof of color vision in animals. Insect species differ in the combinations of photoreceptors in their eyes, with peak sensitivities in ultraviolet (UV) and/or blue, green, and sometimes red parts of the spectrum. The number of photoreceptor spectral types can be as little as one or two, as in the grasshopper Phlaeoba and the beetle Tribolium, and as many as 10 and more in some species of butterflies and dragonflies. However, not all spectral receptor types are necessarily used for color vison. For example, the butterfly Papilio xuthus uses only four of its eight photoreceptors for color vision. Some insects have separate channels for processing chromatic and achromatic (lightness) information. In the honeybee, the achromatic channel has high spatial resolution and is mediated only by long-wavelength sensitive, or “green,” photoreceptors alone, whereas the spatial resolution of chromatic vision is low and mediated by all three spectral types of photoreceptors. Whether other insects have a similar separation of chromatic and achromatic vision remains uncertain. In contrast to vertebrates, insects do not use distinct sets of photoreceptors for nocturnal vision, and some nocturnal insects can see color at night. Insect photoreceptors are inherently polarization sensitive because of their microvillar organization. Therefore, some insects cannot discriminate changes in polarization of light from changes in its spectral composition. However, many insects sacrifice polarization sensitivity to retain reliable color vision. For example, in the honeybee, polarization sensitivity is eliminated by twisting the rhabdom in most parts of its compound eye except for the dorsal rim area that is specialized in polarization vision. Insects experience color constancy and color-contrast phenomena. Although in humans these aspects of vision are often attributed to cortical processing of color, simple models based on photoreceptor adaptation may explain color constancy and color induction in insects. Color discriminations can be evaluated using a simple model, which assumes that it is limited by photoreceptor noise. This model can help to predict discrimination of colors that are ecologically relevant, such as flower colors for pollinating insects. However, despite the fact that many insects forage on flowers, there is no evidence that insect pollinator vision coevolved with flower colors. The diverse color vision in butterflies appears to adaptively facilitate the recognition of their wing colors.

Article

Insect Navigation: Neural Basis to Behavior  

Stanley Heinze

Navigation is the ability of animals to move through their environment in a planned manner. Different from directed but reflex-driven movements, it involves the comparison of the animal’s current heading with its intended heading (i.e., the goal direction). When the two angles don’t match, a compensatory steering movement must be initiated. This basic scenario can be described as an elementary navigational decision. Many elementary decisions chained together in specific ways form a coherent navigational strategy. With respect to navigational goals, there are four main forms of navigation: explorative navigation (exploring the environment for food, mates, shelter, etc.); homing (returning to a nest); straight-line orientation (getting away from a central place in a straight line); and long-distance migration (seasonal long-range movements to a location such as an overwintering place). The homing behavior of ants and bees has been examined in the most detail. These insects use several strategies to return to their nest after foraging, including path integration, route following, and, potentially, even exploit internal maps. Independent of the strategy used, insects can use global sensory information (e.g., skylight cues), local cues (e.g., visual panorama), and idiothetic (i.e., internal, self-generated) cues to obtain information about their current and intended headings. How are these processes controlled by the insect brain? While many unanswered questions remain, much progress has been made in recent years in understanding the neural basis of insect navigation. Neural pathways encoding polarized light information (a global navigational cue) target a brain region called the central complex, which is also involved in movement control and steering. Being thus placed at the interface of sensory information processing and motor control, this region has received much attention recently and emerged as the navigational “heart” of the insect brain. It houses an ordered array of head-direction cells that use a wide range of sensory information to encode the current heading of the animal. At the same time, it receives information about the movement speed of the animal and thus is suited to compute the home vector for path integration. With the help of neurons following highly stereotypical projection patterns, the central complex theoretically can perform the comparison of current and intended heading that underlies most navigation processes. Examining the detailed neural circuits responsible for head-direction coding, intended heading representation, and steering initiation in this brain area will likely lead to a solid understanding of the neural basis of insect navigation in the years to come.

Article

Integration of Peripheral and Central Systems in Control of Ingestive and Reproductive Behavior  

Jill E. Schneider

During the evolution of animals, survival and reproduction depended upon mechanisms that maintained internal homeostasis in the face of environmental change. These environmental changes included fluctuations in ambient temperature, food availability, humidity, day length, and population density. Most, if not all, of these variables have effects on the availability of energy, and most vertebrate species have mechanisms that sense energy availability and adjust behavioral priorities accordingly. For example, when the availability of food and potential mating partners is stable and abundant, brain mechanisms often inhibit ingestive behavior, increase energy expenditure, and give priority to courtship and mating. In response to severe energy shortages, brain mechanisms are likely to stimulate foraging, food hoarding, and overeating. These same deficits often delay reproductive development or inhibit adult reproductive behavior. Such adaptations involve the integration of sensory signals with peripheral hormone signals and central effectors, and they are key to understanding health and disease, particularly obesity, eating disorders, and diabetes. The link between energy balance and reproduction recurs repeatedly, whether in the context of the sensory-somatic system, the autonomic nervous system, or the neuroendocrine cascades. Peripheral signals that are detected by receptors on vagal and splanchnic nerves are relayed to the caudal hindbrain. This brain area contains the effectors for peripheral hormone secretion and for chewing and swallowing, and this same brain area contains receptors for humoral and metabolic signals from peripheral circulation. The caudal hindbrain is therefore a strong candidate for integration of multiple signals that control the initiation of meals, meal size, energy storage, and energy expenditure, including the energy expended on reproduction. There are some differences between the reproductive and ingestive mechanisms, but there are also many striking similarities. There are still gaps in our knowledge about the nature and location of metabolic receptors and the pathways to their effectors. Some of the most promising research is designed to shed light on how hormonal signals might be enhanced or modulated by the peripheral energetic condition (e.g., the level of oxidizable metabolic fuels).

Article

The Interaction of Perception and Memory  

Emma Megla and Wilma A. Bainbridge

Whereas visual perception is the interpretation of the light that enters the retina of the eye, long-term memory is the encoding, storage, and retrieval of perceptual experiences and learned information. Although these are separable processes, they continuously interact and influence each other. For example, the underlying perceptual features of an image result in large consistency in whether people will remember or forget it, and the visual similarities that images share can influence how well they will be remembered. The exaggeration of visual features, such as enlarged eyes on a face, can lead to enhanced memory, and a buildup in perceptual experience can also improve memory. In addition to perception influencing memory, memory also influences perception. Familiarity with an object or object category can result in enhanced perceptual processing, or even lead to the stimuli “looking” different from how they otherwise would. Additionally, learning a new category of objects changes how we perceive its categorical members, and even members of different, related categories. Perception and memory are closely intertwined in the brain as well, with mechanisms that allow similar perceptual items to be distinguished in memory, but also support incomplete perceptual details to be filled in from memory. Additionally, there are divisions in the brain dedicated to the perceptual and mnemonic processing of different object categories, such as faces and scenes. In other words, there are widespread examples in which memory and perception influence each other, with neural mechanisms and areas set in place to deal with these complex interactions.

Article

Invertebrate Nociception  

Nathaniel J. Himmel, Atit A. Patel, and Daniel N. Cox

Nociception is a protective mechanism that mediates behavioral responses to a range of potentially damaging stimuli, including noxious temperature, chemicals, and mechanical stimulation. Nociceptive mechanisms are found throughout metazoans. Noxious stimuli are transduced by specialized, high-threshold peripheral nociceptors, which fire action potentials to elicit adaptive behavioral responses. Nociception is essential for survival and provides a mechanism for sensory perception of noxious stimuli, which alerts the organism to potential environmental dangers. When coupled with pain sensation and complex behavioral responses, this mechanism protects the organism from incipient damage. Moreover, acute and chronic pain may manifest as altered nociception in neuropathic pain states. Elucidating the neural bases of nociception is therefore important for identifying and implementing novel strategies for the treatment of neuropathic pain, as well as uncovering the mechanistic bases by which the nervous system integrates information to produce specific behaviors in response to a range of noxious stimuli. Invertebrate organisms, such as Drosophila melanogaster and Caenorhabditis elegans, have emerged as powerful, genetically tractable platforms for exploring these questions. Here, we concisely review the current state of knowledge regarding the cells, molecules, neural circuits, and behaviors associated with invertebrate nociception in the fruit fly and nematode worm.

Article

Investigating Learning and Memory in Humans  

Evangelia G. Chrysikou, Elizabeth Espinal, and Alexandra E. Kelly

Memory refers to the set of cognitive systems and the neural structures that support them that allow humans to learn from experience, leverage this knowledge to understand and guide behavior in the present, and use past memories to think about and plan for the future. Neuroscience research on learning and memory has leveraged advances in behavioral methods, structural and functional brain imaging, noninvasive brain stimulation, and lesion studies to evaluate synergies and dissociations among small- and large-scale neural networks in support of memory performance. Overall, this work has converged to a conceptualization of new memories as representations of distributed patterns of neural activity across cortical and subcortical brain systems that provide neural grounding of sensorimotor and perceptual experiences, actions, thoughts, and emotions, and which can be reinstated as a result of internal or external cues. Most of this literature has supported dissociations among working and long-term memory, as well as between procedural, episodic, and semantic memories. On the other hand, progress in human neuroscience methodologies has revealed the interdependence of these memory systems in the context of complex cognitive tasks and suggests a dynamic and highly interactive neural architecture underlying human learning and memory. Future neuroscience research is anticipated to focus on understanding the neural mechanisms supporting this interactivity at the cellular and systems levels, as well as investigating the time course of their engagement.

Article

Jean-Martin Charcot (1825–1893)  

Olivier Walusinski

Jean-Martin Charcot (1825–1893), son of a Parisian craftsman, went on to a brilliant university career and worked his way to the top of the hospital hierarchy. Becoming a resident in 1858 at the women’s nursing home and asylum at La Salpêtrière Hospital, he returned there in 1868 as chief physician. Observing more than 2,000 elderly women, he first worked as a geriatrician–internist, leading him to describe thyroid pathology, cruoric pulmonary embolism, and so forth. To deal with the numerous nervous system pathologies, he applied the anatomoclinical method with the addition of microscopy. In less than around 10 years, his perspicacious clinical eye enabled him to describe Parkinson’s disease, multiple sclerosis, amyotrophic lateral sclerosis, and tabetic arthropathy and to identify medullary localizations, for example. Already aware of functional neurological disorders, at that time referred to as hysteria and frequent to this day, Charcot used hypnosis to try to decipher the pathophysiology. His thinking gradually evolved from looking for lesions to recognizing triggering psychological trauma. This prolonged search, misinterpreted for years, opened the way to fine, precise clinical semiology, specific to neurology and psychosomatic medicine. Charcot knew how to surround himself with a cohort of brilliant clinicians, who often became as famous as he was, notably Pierre Marie (1853–1940), Georges Gilles de la Tourette (1857–1904), Joseph Babiński (1857–1932), and Pierre Janet (1859–1947). This cohort and the breadth of Charcot’s innovative work define what is now classically called the “Salpêtrière School.”

Article

Jean-René Cruchet (1875–1959)  

Olivier Walusinski

Jean-René Cruchet (1875–1959) was a French physician from Bordeaux, where he practiced for the entirety of his career. His notoriety resulted from his publication of the first cases of the encephalitis lethargica epidemic in World War I soldiers in 1917, a few days before Constantin von Economo reported his cases. Cruchet developed an interest in abnormal movements, notably tics and dystonia, for which he primarily saw a psychological cause, to be treated rigorously with good habits and repressive precepts. He wrote prolifically about his areas of interest, also focusing on parkinsonian syndromes and the treatment of hysterics, notably soldiers with camptocormia. One of the first physicians to also be an aviator, Cruchet was a pioneer in the study of autonomic modifications caused by flying and pressure variations, which he referred to as aviator’s disease. As a personality with an outsized ego, he imagined that he would remain as famous after his death as Jean-Martin Charcot or Louis Pasteur.

Article

Jellyfish Locomotion  

Richard Satterlie

Two dichotomies exist within the swim systems of jellyfish—one centered on the mechanics of locomotion and the other on phylogenetic differences in nervous system organization. For example, medusae with prolate body forms use a jet propulsion mechanism, whereas medusae with oblate body forms use a drag-based marginal rowing mechanism. Independent of this dichotomy, the nervous systems of hydromedusae are very different from those of scyphomedusae and cubomedusae. In hydromedusae, marginal nerve rings contain parallel networks of neurons that include the pacemaker network for the control of swim contractions. Sensory structures are similarly distributed around the margin. In scyphomedusae and cubomedusae, the swim pacemakers are restricted to marginal integration centers called rhopalia. These ganglionlike structures house specialized sensory organs. The swim system adaptations of these three classes (Hydrozoa, Scyphozoa, and Cubozoa), which are constrained by phylogenetics, still adhere to the biomechanical efficiencies of the prolate/oblate dichotomy. This speaks to the adaptational abilities of the cnidarian nervous system as specialized in the medusoid forms.

Article

Leech Behavioral Choice  

William B. Kristan Jr.

New techniques for recording the activity of many neurons simultaneously have given insights into how neuronal circuits make the decision to perform one of many possible behaviors. A long-standing hypothesis for how behavioral choices are made in any animal is that “command neurons” are responsible for selecting individual behaviors, and that these same neurons inhibit the command neurons that elicit other behaviors. In fact, this mechanism has turned out to be just one of several ways that such decision-making is accomplished. In particular, for some behavioral choices, the circuits appear to overlap, sometimes extensively, to perform two or more behaviors. Making decisions using such “multifunctional neurons” has been proposed for large neural networks, but this strategy appears to be used in relatively small nervous systems, too. These simpler nervous systems can serve as useful test systems to test hypotheses about how the dynamics of networks of neurons can be used to select among different behaviors, similar to the mechanisms used by leeches deciding to swim, shorten, crawl, or feed.

Article

Leech Mechanosensation  

Brian D. Burrell

The medicinal leech (Hirudo verbana) is an annelid (segmented worm) and one of the classic model systems in neuroscience. It has been used in research for over 50 years and was one of the first animals in which intracellular recordings of mechanosensory neurons were carried out. Remarkably, the leech has three main classes of mechanosensory neurons that exhibit many of the same properties found in vertebrates. The most sensitive of these neurons are the touch cells, which are rapidly adapting neurons that detect low-intensity mechanical stimuli. Next are the pressure cells, which are slow-adapting sensory neurons that respond to higher intensity, sustained mechanostimulation. Finally, there are nociceptive neurons, which have the highest threshold and respond to potentially damaging mechanostimuli, such as a pinch. As observed in mammals, the leech has separate mechanosensitive and polymodal nociceptors, the latter responding to mechanical, thermal, and chemical stimuli. The cell bodies for all three types of mechanosensitive neurons are found in the central nervous system where they are arranged as bilateral pairs. Each neuron extends processes to the skin where they form discrete receptive fields. In the touch and pressure cells, these receptive fields are arranged along the dorsal-ventral axis. For the mechano-only and polymodal nociceptive neurons, the peripheral receptive fields overlap with the mechano-only nociceptor, which also innervates the gut. The leech also has a type of mechanosensitive cell located in the periphery that responds to vibrations in the water and is used, in part, to detect potential prey nearby. In the central nervous system, the touch, pressure, and nociceptive cells all form synaptic connections with a variety of motor neurons, interneurons, and even each other, using glutamate as the neurotransmitter. Synaptic transmission by these cells can be modulated by a variety of activity-dependent processes as well as the influence of neuromodulatory transmitters, such as serotonin. The output of these sensory neurons can also be modulated by conduction block, a process in which action potentials fail to propagate to all the synaptic release sites, decreasing synaptic output. Activity in these sensory neurons leads to the initiation of a number of different motor behaviors involved in locomotion, such as swimming and crawling, as well as behaviors designed to recoil from aversive/noxious stimuli, such as local bending and shortening. In the case of local bending, the leech is able to bend in the appropriate direction away from the offending stimuli. It does so through a combination of which mechanosensory cell receptive fields have been activated and the relative activation of multiple sensory cells decoded by a layer of downstream interneurons.

Article

Long-Term Potentiation and Long-Term Depression  

Arianna Maffei

Synaptic connections in the brain can change their strength in response to patterned activity. This ability of synapses is defined as synaptic plasticity. Long lasting forms of synaptic plasticity, long-term potentiation (LTP), and long-term depression (LTD), are thought to mediate the storage of information about stimuli or features of stimuli in a neural circuit. Since its discovery in the early 1970s, synaptic plasticity became a central subject of neuroscience, and many studies centered on understanding its mechanisms, as well as its functional implications.

Article

Magnetoreception and Bird Navigation  

Roswitha Wiltschko and Wolfgang Wiltschko

The magnetic field of the earth provides birds with navigational information, with birds having two different receptor systems, one for the direction, the other for the intensity of the geomagnetic field. The direction of the geomagnetic field is used as a compass, with the avian magnetic compass being an inclination compass not recording the polarity of the field. The respective directional information is perceived by light-dependent radical pair processes in the eyes, with cryptochrome, a photopigment with the chromophore flavin adenine dinucleotide as receptor molecule. It is transmitted by the optic nerve to the brain, where it is processed by parts of the visual system. The magnetic compass not only serves to orient avian flights but also acts as a reference system for route reversal, calibrating the astronomical compass systems, and in migratory birds, as reference for the innate information on the migratory direction. Magnetic intensity and inclination that show gradients from the poles to the magnetic equator are part of the mechanisms that allow birds to determine their position. Intensity is perceived by receptors based on magnetite, a permanently magnetic material. The effect of a brief, strong magnetic pulse and its duration indicates that superparamagnetic particles are involved. The respective information is transmitted by the ophthalmic branch of the trigeminal nerve to the trigeminal brainstem complex in the brain. Testing birds in magnetic fields of a distant site, i.e., magnetically simulating a displacement, documents that magnetic intensity and inclination are very most important components of the navigational “map” that enables birds to determine their position relative to the goal and thus derive the compass course leading to this goal. Furthermore, certain magnetic conditions act as signposts and elicit specific spontaneous responses.

Article

Mammalian Visual System Organization  

Farran Briggs

Many mammals, including humans, rely primarily on vision to sense the environment. While a large proportion of the brain is devoted to vision in highly visual animals, there are not enough neurons in the visual system to support a neuron-per-object look-up table. Instead, visual animals evolved ways to rapidly and dynamically encode an enormous diversity of visual information using minimal numbers of neurons (merely hundreds of millions of neurons and billions of connections!). In the mammalian visual system, a visual image is essentially broken down into simple elements that are reconstructed through a series of processing stages, most of which occur beneath consciousness. Importantly, visual information processing is not simply a serial progression along the hierarchy of visual brain structures (e.g., retina to visual thalamus to primary visual cortex to secondary visual cortex, etc.). Instead, connections within and between visual brain structures exist in all possible directions: feedforward, feedback, and lateral. Additionally, many mammalian visual systems are organized into parallel channels, presumably to enable efficient processing of information about different and important features in the visual environment (e.g., color, motion). The overall operations of the mammalian visual system are to: (1) combine unique groups of feature detectors in order to generate object representations and (2) integrate visual sensory information with cognitive and contextual information from the rest of the brain. Together, these operations enable individuals to perceive, plan, and act within their environment.