Helen Sherk

The Claustrum and the Cerebral Cortex

For nearly a century, the identity and function of the claustrum have presented a puzzle. Its location adjacent to the putamen suggests that it belongs to the basal ganglia, but its afferent and efferent connections are mainly with the cortex, implying that, despite its subcortical location, the claustrum deals primarily with cortical information.

A reassessment of the lower visual field map in striate-recipient lateral suprasylvian cortex.

Lateral suprasylvian visual cortex in the cat has been studied extensively, but its retinotopic organization remains controversial. Although some investigators have divided this region into many distinct areas, others have argued for a simpler organization. A clear understanding of the regions retinotopic organization is important in order to define distinct areas that are likely to subserve unique visual functions. We therefore reexamined the map of the lower visual field in the striate-recipient region of lateral suprasylvian cortex, a region we refer to as the lateral suprasylvian area, LS. A dual mapping approach was used. First, receptive fields were plotted at numerous locations along closely spaced electrode penetrations; second, different anterograde tracers were injected at retinotopically identified sites in area 17, yielding patches of label in LS. To visualize the resulting data, suprasylvian cortex was flattened with the aid of a computer. Global features of the map reported in many earlier studies were confirmed. Central visual field was represented posteriorly, and elevations generally shifted downward as one moved anteriorly. Often (though not always) there was a progression from peripheral locations towards the vertical meridian as the electrode moved down the medial suprasylvian bank. The map had some remarkable characteristics not previously reported in any map in the cat. The vertical meridians representation was split into two pieces, separated by a gap, and both pieces were partially internalized within the map. Horizontal meridian occupied the gap. The area centralis usually had a dual representation along the posterior boundary of the lower field representation, and other fragments of visual field were duplicated as well. Finally, magnification appeared to change abruptly and unexpectedly, so that compressed regions of representation adjoined expanded regions. Despite its complexity, we found the map to be more orderly than previously thought. There was no clearcut retinotopic basis on which to subdivide LSs lower field representation into distinct areas.

The use of visual information for planning accurate steps in a cluttered environment

When an observer walks across irregular terrain, he uses vision to plan his steps. How far in advance of each step does he acquire the critical information? We trained cats to walk accurately down a cluttered alley, and then turned out the light in mid-trial. Cats usually continued to walk without error for one to four steps, indicating that they had acquired the information to guide each step well before foot contact.

Gaze during visually-guided locomotion in cats.

Visual guidance is often critical during locomotion. To understand how the visual system performs this function it is necessary to know what pattern of retinal image motion neurons experience. If a locomoting observer maintains an angle of gaze that is constant relative to his body, retinal image motion will resemble Gibsons (The Perception of the Visual World (1950)) well-known optic flow field. However, if a moving observer fixates and tracks a stationary feature of the environment, or shifts his gaze, retinal motion will be quite different. We have investigated gaze in cats during visually-guided locomotion. Because cats generally maintain their eyes centered in the orbits, their gaze can be monitored with reasonable accuracy by monitoring head position. Using a digital videocamera, we recorded head position in cats as they walked down a cluttered alley. For much of the time, cats maintained a downward angle of gaze that was constant relative to their body coordinates; these episodes averaged 240 ms in duration and occupied 48-71% of the total trial time. Constant gaze episodes were separated by gaze shifts, which often coincided with blinks. Only rarely did we observe instances when cats appeared to fixate and track stationary features of the alley. We hypothesize that walking cats acquire visual information primarily during episodes of constant gaze, when retinal image motion resembles Gibsons conventional optic flow field.

The retinotopic match between area 17 and its targets in visual suprasylvian cortex.

SummaryIt is widely believed that cells in area 17 send axons specifically to neurons in other cortical areas whose receptive fields coincide with their own. We asked whether this was true in cats for area 17s projection to a large suprasylvian visual area, the Clare-Bishop area. Receptive fields were plotted at multiple sites in the Clare-Bishop area. Then, in area 17, anterograde tracer was injected at a retinotopically-characterized site, giving rise to patches of labeled terminals in the Clare-Bishop area. Receptive field centers recorded within these patches were located close to the visual field location at the injection site in area 17. Receptive fields recorded outside of labeled patches, on the other hand, were never in register with that plotted in area 17. However, due to their large size, even fields located outside of labeled patches often encompassed the visual field point injected in area 17. In other experiments, receptive fields for both neurons and presumed cortical afferents were recorded at the same site in the Clare-Bishop area. The centers of such pairs of receptive fields were on average less than 1° apart. Finally, the gaps between widely separated patches of label were investigated. Both physiological and anatomical evidence indicated that a different part of the visual field was represented in gaps than in the adjacent patches.

Neuronal responses in extrastriate cortex to objects in optic flow fields

During locomotion, observers respond to objects in the environment that may represent obstacles to avoid or landmarks for navigation. Although much is known about how visual cortical neurons respond to stimulus objects moving against a blank background, nothing is known about their responses when objects are embedded in optic flow fields (the patterns of motion seen during locomotion). We recorded from cells in the lateral suprasylvian visual area (LS) of the cat, an area probably analogous to area MT. In our first experiments, optic flow simulations mimicked the view of a cat trotting across a plain covered with small balls; a black bar lying on the balls served as a target object. In subsequent experiments, optic flow simulations were composed of natural elements, with target objects representing bushes, rocks, and variants of these. Cells did not respond to the target bar in the presence of optic flow backgrounds, although they did respond to it in the absence of a background. However, 273/423 cells responded to at least one of the taller, naturalistic objects embedded in optic flow simulations. These responses might represent a form of image segmentation, in that cells detected objects against a complex background. Surprisingly, the responsiveness of cells to objects in optic flow fields was not correlated with preferred direction as measured with a moving bar or whole-field texture. Because the direction of object motion was determined solely by receptive-field location, it often differed considerably from a cells preferred direction. About a quarter of the cells responded well to objects in optic flow movies but more weakly or not at all to bars moving in the same direction as the object, suggesting that the optic flow background modified or suppressed direction selectivity.

Visual response properties of cortical inputs to an extrastriate cortical area in the cat

The existence of multiple areas of extrastriate visual cortex raises the question of how the response properties of each area are derived from its visual input. This question was investigated for one such area in the cat, referred to here as the Clare-Bishop area (Hubel & Wiesel, 1969); it is the region of lateral suprasylvian cortex that receives input from area 17. A novel approach was used, in which kainic acid was injected locally into the Clare-Bishop area, making it possible to record directly from afferent inputs. The response properties of the great majority of a sample of 424 presumed afferents resembled cells in areas 17 and 18. Thus, a systematic comparison was made with cells from area 17s upper layers, the source of its projection to the Clare-Bishop area (Gilbert & Kelly, 1975), to see whether these afferents had distinctive properties that might distinguish them from cells projecting to areas 18 or 19. Some differences did emerge: (1) The smallest receptive fields typical of area 17 were relatively scarce among afferents. (2) Direction-selective afferents were more abundant than were such cells in area 17. (3) End-stopped afferents were extremely rare, although end-stopped cells were common in area 17s upper layers. Despite these differences, afferents were far more similar in their properties to cells in areas 17 and 18 than to cells in the Clare-Bishop area. Compared to the latter, afferents showed major discrepancies in receptive-field size, in direction selectivity, in end-stopping, and in ocular dominance distribution. These differences seem most likely to stem from circuitry intrinsic to the Clare-Bishop area.

Flattening the cerebral cortex by computer

A computer program was developed for unfolding the cerebral cortex so that it could be viewed as a 2-dimensional surface. Input to the program consisted of tissue sections cut in a standard plane of section. Each section was represented by one line, which corresponded to a contour line in the flattened map. From these data, the computer constructed a 3-dimensional surface representation, which it then flattened. Because the cerebral cortex has considerable intrinsic curvature, flattening required that some regions be expanded and others shrunken. These changes occurred as a natural consequence of local decisions made by the computer as it laid down successive contours. The user could intervene during both surfacing and flattening in order to shape the developing map. The program has been used to generate 37 flattened maps from various regions of cat cortex, and 1 from monkey cortex. The local topography of cortical features such as gyri, sulci, architectonic boundaries, and patches of transported tracer, appeared to be conserved fairly faithfully. Areal distortion was also modest, with an average change in surface area of only 12%.

A comparison of magnification functions in area 19 and the lateral suprasylvian visual area in the cat

A retinotopic map can be described by a magnification function that relates magnification factor to visual field eccentricity. Magnification factor for primary visual cortex (VI) in both the cat and the macaque monkey is directly proportional to retinal ganglion cell density. However, among those extrastriate areas for which a magnification function has been described, this is often not the case. Deviations from the pattern established in V1 are of considerable interest because they may provide insight into an extrastriate areas role in visual processing. The present study explored the magnification function for the lateral suprasylvian area (LS) in the cat. Because of its complex retinotopic organization, magnification was calculated indirectly using the known magnification function for area 19. Small tracer injections were made in area 17, and the extent of anterograde label in LS and in area 19 was measured. Using the ratio of cortical area labeled in LS to that in area 19, and the known magnification factor for area 19 at the corresponding retinotopic location, we were able to calculate magnification factor for LS. We found that the magnification function for LS differed substantially from that for area 19: central visual field was expanded, and peripheral field compressed in LS compared with area 19. Additionally, we found that the lower vertical meridians representation was compressed relative to that of the horizontal meridian. We also examined receptive field size in areas 17, 19, and LS and found that, for all three areas, receptive field size was inversely proportional to magnification factor.

A novel system for recording from single neurons in unrestrained animals.

To observe neural activity in animals engaged in natural behavior, it is often desirable to minimize or eliminate restraint of the animal. We have developed a simple system for recording from single units in unrestrained cats. An implant with multiple guide tubes and a tiny microdrive is placed inside the recording chamber. An indwelling Pt-Ir microelectrode is advanced incrementally during recording sessions that occur over a period of weeks or months. Electrodes can be easily replaced. We obtain excellent recording stability, and also have been able to sample extensively from a region of cortex or brain stem in a single animal. The essential electronics have been miniaturized and sewn into a light-weight walking jacket, so that we can collect data from a cat who is not connected to any fixed equipment.