Coding of visual space by premotor neurons.

In primates, the premotor cortex is involved in the sensory guidance of movement. Many neurons in ventral premotor cortex respond to visual stimuli in the space adjacent to the hand or arm. These visual receptive fields were found to move when the arm moved but not when the eye moved; that is, they are in arm-centered, not retinocentric, coordinates. Thus, they provide a representation of space near the body that may be useful for the visual control of reaching.

Coding the locations of objects in the dark.

The ventral premotor cortex in primates is thought to be involved in sensory-motor integration. Many of its neurons respond to visual stimuli in the space near the arms or face. In this study on the ventral premotor cortex of monkeys, an object was presented within the visual receptive fields of individual neurons, then the lights were turned off and the object was silently removed. A subset of the neurons continued to respond in the dark as if the object were still present and visible. Such cells exhibit "object permanence," encoding the presence of an object that is no longer visible. These cells may underlie the ability to reach toward or avoid objects that are no longer directly visible.

Visual receptive fields of neurons in inferotemporal cortex of the monkey.

Neurons in inferotemporal cortex (area TE) of the monkey had visual receptive fields which were very large (greater than 10 by 10 degrees) and almost always included the fovea. Some extended well into both halves of the visual field, while others were confined to the ipsilateral or contralateral side. These neurons were differentially sensitive to several of the following dimensions of the stimulus: size and shape, color, orientation, and direction of movement.

Auditory association cortex lesions impair auditory short-term memory in monkeys.

Monkeys that were trained to perform auditory and visual short-term memory tasks (delayed matching-to-sample) received lesions of the auditory association cortex in the superior temporal gyrus. Although visual memory was completely unaffected by the lesions, auditory memory was severely impaired. Despite this impairment, all monkeys could discriminate sounds closer in frequency than those used in the auditory memory task. This result suggests that the superior temporal cortex plays a role in auditory processing and retention similar to the role the inferior temporal cortex plays in visual processing and retention.

Neurogenesis in the neocortex of adult primates.

In primates, prefrontal, inferior temporal, and posterior parietal cortex are important for cognitive function. It is shown that in adult macaques, new neurons are added to these three neocortical association areas, but not to a primary sensory area (striate cortex). The new neurons appeared to originate in the subventricular zone and to migrate through the white matter to the neocortex, where they extended axons. These new neurons, which are continually added in adulthood, may play a role in the functions of association neocortex.

Huxley versus Owen: the hippocampus minor and evolution.

In Victorian Britain a major debate over evolution raged between Thomas H. Huxley and Richard Owen. The central issue between them was whether or not the human brain was unique in having a hippocampus minor (also known as the calcar avis), a posterior horn and a posterior lobe.

'Psychosurgery' in renaissance art.

Hieronymus Bosch and other early Renaissance artists depicted 'stone operations' in which stones were supposedly surgically removed from the head as a treatment for mental illness. These works have usually been interpreted either as portraying a contemporary practice of medical charlatans or as an allegory of human folly, rather than a real event. As trepanation for head injury and mental disease was actually carried out in Europe at this time, another interpretation of these works is that they are derived from a common medical practice of the day.

Rembrandt's 'The anatomy lesson of Dr. Joan Deijman'.

Rembrandt's striking painting of a human brain being dissected by a headless figure is actually a fragment of a larger work. The original was both a commissioned group portrait of a surgeons' guild and an account of a public dissection. Such dissections served both educational and entertainment functions in 17th century Holland.

Face recognition.

The study of face-selective neurons in the monkey temporal lobe, and face recognition deficits in humans after brain damage have both become very active fields of investigation. Face-selective neurons appear to be members of ensembles for coding faces rather than individual face detectors or grandmother cells. They reflect the more general role of temporal cortex in pattern recognition. In humans there are a variety of face-processing impairments that result from damage to different areas, and which reflect interference at different levels of processing of the facial image.

Spatial maps for the control of movement.

Neurons in the ventral premotor cortex of the monkey encode the locations of visual, tactile, auditory and remembered stimuli. Some of these neurons encode the locations of stimuli with respect to the arm, and may be useful for guiding movements of the arm. Others encode the locations of stimuli with respect to the head, and may be useful for guiding movements of the head. We suggest that a general principle of sensory-motor integration is that the space surrounding the body is represented in body-part-centered coordinates. That is, there are multiple coordinate systems used to guide movement, each one attached to a different part of the body. This and other recent evidence from both monkeys and humans suggest that the formation of spatial maps in the brain and the guidance of limb and body movements do not proceed in separate stages but are closely integrated in both the parietal and frontal lobes.

Temperature effects on the resetting of the phase of the Neurospora circadian rhythm.

Various temperatures relative to a 25 degrees C control have been applied as phase-resetting agents in release-assay experiments using the conidiation rhythm of the mold Neurospora crassa. The larger the difference in temperature from the 25 degrees C control, the stronger the phase-resetting effects. Phase-resetting curves of the weak type (type 1) are observed for temperatures up to 28 degrees C and down to 22 degrees C, whereas temperatures above 28 degrees C and less than 22 degrees C generally cause phase-resetting curves of the strong type (type 0). Singularity behavior occurs at approximately 22 degrees C and 28 degrees C when 25 degrees C is used as the control temperature. When a different control of 29.5 degrees C is used in a release-assay experiment and the resetting temperature is 25 degrees C, near-singularity behavior is observed.

Visual topography of V2 in the macaque.

The representation of the visual field in the area adjacent to striate cortex was mapped with multiunit electrodes in the macaque. The animals were immobilized and anesthetized and in each animal 30 to 40 electrode penetrations were typically made over several recording sessions. This area, V2, contains a topographically organized representation of the contralateral visual field up to an eccentricity of at least 80 degrees. The representation of the vertical meridian is adjacent to that in striate cortex (V1) and forms the posterior border of V2. The representation of the horizontal meridian in V2 forms the anterior border of V2 and is split so that the representation of the lower visual field is located dorsally and that of the upper field ventrally. As in V1, the representation of the central visual field is magnified relative to that of the periphery. The area of V2 is slightly smaller than that of V1. At a given eccentricity, receptive field size in V2 is larger than in V1. The myeloarchitecture of V2 is distinguishable from that of the surrounding cortex. The location of V2 corresponds, at least approximately, to that of cytoarchitectonic Area OB. V2 is bordered anteriorly by several other areas containing representations of the visual field.

Topographical organization of cortical afferents to extrastriate visual area PO in the macaque: a dual tracer study.

We have examined the origin and topography of cortical projections to area PO, an extrastriate visual area located in the parieto-occipital sulcus of the macaque. Distinguishable retrograde fluorescent tracers were injected into area PO at separate retinotopic loci identified by single-neuron recording. The results indicate that area PO receives retinotopically organized inputs from visual areas V1, V2, V3, V4, and MT. In each of these areas the projection to PO arises from the representation of the periphery of the visual field. This finding is consistent with neurophysiological data indicating that the representation of the periphery is emphasized in PO. Additional projections arise from area MST, the frontal eye fields, and several divisions of parietal cortex, including four zones within the intraparietal sulcus and a region on the medial dorsal surface of the hemisphere (MDP). On the basis of the laminar distribution of labeled cells we conclude that area PO receives an ascending input from V1, V2, and V3 and receives descending or lateral inputs from all other areas. Thus, area PO is at approximately the same level in the hierarchy of visual areas as areas V4 and MT. Area PO is connected both directly and indirectly, via MT and MST, to parietal cortex. Within parietal cortex, area PO is linked to particular regions of the intraparietal sulcus including VIP and LIP and two newly recognized zones termed here MIP and PIP. The wealth of connections with parietal cortex suggests that area PO provides a relatively direct route over which information concerning the visual field periphery can be transmitted from striate and prestriate cortex to parietal cortex. In contrast, area PO has few links with areas projecting to inferior temporal cortex. The pattern of connections revealed in this study is consistent with the view that area PO is primarily involved in visuospatial functioning.

Contribution of striate cortex and the superior colliculus to visual function in area MT, the superior temporal polysensory area and the inferior temporal cortex.

We studied the visual responses of single neurons in three extra-striate visual areas of the macaque following lesions of striate cortex, lesions of the tecto-pulvinar system or both. After striate lesions, there was (a) considerable specific activity remaining in area MT including direction selectivity, (b) only non-specific activity in the superior temporal polysensory area (STP), and (c) no visual responsiveness at all in inferior temporal cortex (IT). In animals with striate lesions, interruption of the tecto-pulvinar pathway eliminated the residual visual activity in MT and STP that survived the striate lesions. Interruption of the tecto-pulvinar pathway alone had little or no effect on visual evoked activity in any of the three areas. These results are related to the relative dependence of visual responsiveness in MT, STP and IT on striate cortex and the superior colliculus, to differences between the dorsal and ventral cortical processing streams, and to neural mechanisms underlying blind sight.

Visual discrimination impairments following lesions of the superior temporal sulcus are not specific for facial stimuli.

Six rhesus monkeys took part in an experiment on visual learning. In three of the monkeys the part of the superior temporal sulcus in which many of the cells respond selectively to some aspect of faces was removed, while the remaining three animals served as unoperated controls. In Experiment 1 they learned a series of two-choice visual discriminations between patterns. The animals with lesions of the superior temporal sulcus were markedly impaired. The discriminations were of two types: in the first, the discriminanda differed in shape (e.g. Y and 3), while in the second they differed only in their orientation (e.g. ). Unlike animals with lesions to the neighbouring inferior temporal cortex who are impaired on shape but not orientation discriminations, animals with lesions of the superior temporal sulcus were equally impaired on both types of discrimination. In Experiment 2 the same six animals learned a series of discriminations between discriminanda which consisted of photographs of pairs of eyes. Each discrimination was between a set of eyes which looked directly at the viewer and a set in which the gaze was laterally averted to varying degrees. Again, animals with lesions of the superior temporal sulcus showed a marked impairment. We conclude that this impairment may be a general impairment in two-choice visual discrimination learning, rather than a selective impairment in discrimination of eye gaze. This result warns against a simple interpretation of the function of this area as a "face area", concerned only, or chiefly, with the perception and significance of parts of the body, notably faces, and their movements.

Stimulus equivalence after inferior temporal lesions in monkeys.

Monkeys with inferior temporal cortex lesions and unoperated monkeys were trained to discriminate pairs of objects and then tested for transfer after the original discriminanda were made larger or smaller, rotated, or presented as two-dimensional projections. The two groups of monkeys transferred equally well to the discriminanda altered in size or orientation, but only the unoperated animals transferred to the two-dimensional representation.

Responses of inferior temporal cortex and hippocampal neurons during delayed matching to sample in monkeys (Macaca fascicularis).

Single-unit activity was recorded from inferior temporal (IT) cortex and the hippocampus in 2 macaques trained on auditory-visual and visual-visual delayed matching-to-sample tasks. The main purpose of the study was to compare the response properties of delay neurons between the 2 areas. The authors noted that (a) IT cortex delay activity was usually selective to a particular stimulus, whereas hippocampal delay activity was usually nonselective; (b) the level of delay activity was generally larger in the hippocampus than in IT cortex; and (c) unlike IT cortex delay activity, hippocampal delay activity tended to increase in magnitude as the delay progressed. The authors also examined the functional significance of delay activity and noted a higher probability of encountering a delay neuron when the monkeys were performing 75%-100% correct as compared with 50%-75% correct. The significance of these findings for visual recognition memory is discussed.

The effects of inferior temporal and dorsolateral frontal lesions on serial-order behavior and visual imagery in monkeys.

Four monkeys were trained preoperatively on a serial-order task to respond to a set of five visual stimuli in a fixed sequence independent of their location. They were then given a test of visual imagery in which only two of the five stimuli appeared at a time, and the animals were required to respond to them in the order in which they appeared in the original sequence. The monkeys then received bilateral lesions of either inferior temporal cortex or dorsolateral frontal cortex. Dorsolateral frontal lesions had no effect on either serial-order behavior or visual imagery. In contrast, inferior temporal lesions severely impaired serial-order behavior. Once the serial-order task was relearned, however, the inferior temporal animals were completely normal on the test of visual imagery.

The effects of combined superior temporal polysensory area and frontal eye field lesions on eye movements in the macaque monkey.

We previously found [42] that lesions of the superior temporal polysensory area (STP) cause temporary deficits in the production of eye movements. In order to both define regions participating in the ensuing recovery and to further explore the cortical control of eye movements, we examined the effects of addition of frontal eye field (FEF) lesions to STP lesions, on visual fixation, saccadic eye movements, and smooth pursuit eye movements. Three monkeys received bilateral STP lesions followed by a FEF lesion and as a control, an additional monkey received a bilateral inferior temporal cortex (IT) lesion followed by a FEF lesion. All animals had a profound impairment in foveating the central fixation point. This impairment was completely eliminated by turning on a dim light in the testing chamber. Large neglect-like impairments in making saccades were only seen after combined STP and FEF lesions. Impairments in making smooth pursuit eye movements after combined lesions of STP and FEF were larger than those seen after STP lesions but within the range of deficits that have been reported after FEF lesions alone. The impairment of visual fixation in darkness and the lack of impairment under conditions of dim illumination appear to reflect a specific role for the FEF in spatial orientation in the absence of visual landmarks. The FEF also appears to play a more critical role than STP in smooth pursuit. By contrast, STP and the FEF appear to work cooperatively with respect to the production of saccades. We suggest that cortical oculomotor control can flow either through the midbrain or through the FEF and that the FEF pathway is specifically involved in tasks with a discontiguity between the stimuli and the behavioral response while the midbrain pathways are preferentially involved in more stimulus-driven eye movements.

Visual areas in the temporal cortex of the macaque.

Visual receptive fields and responsiveness of neurons to somesthetic and auditory stimuli were studied in the inferior temporal cortex and adjacent regions of immobilized macaques. Neurons throughout cytoarchitectonic area TE were responsive only to visual stimuli and had large receptive fields that almost always included the center of gaze and usually extended into both visual half-fields. There was no indication of any visuotopic organization within area TE. Neurons in an anterior and in a dorsal portion of TE tended to have larger receptive fields. By contrast, dorsal, ventral and anterior to area TE, units often responded to somesthetic and auditory as well as to visual stimuli. In these regions visual receptive fields were even larger than in TE and often included the entire visual field. Posterior to TE the neurons were exclusively visual and had much smaller receptive fields that were confined to the contralateral visual field and were topographically organized.