Frank H. Baker

Effects of visual deprivation and strabismus on the response of neurons in the visual cortex of the monkey, including studies on the striate and prestriate cortex in the normal animal

Abstract The response characteristics of neurons in the striate and prestriate cortex of normal macaque monkeys and those that had been raised under conditions of form deprivation or strabismus were studied with naturally presented visual stimuli. Most of these neurons were best activated by stimuli that had elongated borders of light and darkness which were moved through the receptive fields of the cells perpendicular to the long axis of the stimulus. The responses depended on the size, shape and orientation of the stimuli. In the normal animals most of the cells could be driven from either eye, although in the striate cortex 23% of the neurons were monocular, and in prestriate cortex there were 4% monocular cells. In both striate and prestriate cortex a small number of neurons did not respond to visual stimuli and others did not require an oriented-edge stimulus. Binocular stimulation of binocular cells revealed two classes of neurons. In the first class were those cells that gave a binocular response almost equivalent to the monocular response from the dominant eye and for which variation in the relative position (the ocular disparity) of the stimuli on the two retinas produced little change in the response. This type of neuron was seen in both striate and prestriate cortex. In the second class of binocular cell, seen only in prestriate cortex, the number of action potentials evoked by the stimuli depended on ocular disparity, and for a majority of these cells the response was markedly facilitated over a narrow range of disparities. The form deprived and strabismic animals had undergone eye surgery shortly after birth, producing monocular esotropia, exotropia or form deprivation by closure of the lid of one eye. The acuity in both eyes of each animal was tested during adult life. In general, the neurophysiological results demonstrated changes at the single neuron level which paralleled the degree of the behavioral changes: in animals with very poor acuity in the deprived or deviate eye (the form deprived and one esotropic animal), only a small number of neurons were driven from the operated eye; in one esotrope and the exotropic animal, which had good acuity in the operated eye, many neurons received an input from that eye, but only a small number of these were binocular. Finally, histological examination of the lateral geniculate nuclei of the form deprived animals revealed smaller, paler staining cells in those layers connected with the deprived eye.

Oculomotor strategies for the direction of gaze tested with a real-world activity.

Laboratory-based models of oculomotor strategy that differ in the amount and type of top-down information were evaluated against a baseline case of random scanning for predicting the gaze patterns of subjects performing a real-world activity--walking to a target. Images of four subjects eyes and field of view were simultaneously recorded as they performed the mobility task. Offline analyses generated movies of the eye on scene and a categorization scheme was used to classify the locations of the fixations. Frames from each subjects eye-on-scene movie served as input to the models, and the location of each models predicted fixations was classified using the same categorization scheme. The results showed that models with no top-down information (visual salience model) or with only coarse feature information performed no better than a random scanner; the models ordered fixation locations (gaze pattern) matched less than a quarter of the subjects gaze patterns. A model that used only geographic information outperformed the random scanner and matched approximately a third of the gaze patterns. The best performance was obtained from an oculomotor strategy that used both coarse feature and geographic information, matching nearly half the gaze patterns (48%). Thus, a model that uses top-down information about a targets coarse features and general vicinity does a fairly good job predicting fixation behavior, but it does not fully specify the gaze pattern of a subject walking to a target. Additional information is required, perhaps in the form of finer feature information or knowledge of a tasks procedure.

Spatial and chromatic properties of neurons subserving foveal and parafoveal vision in rhesus monkey.

The response properties of neurons in the region of striate cortex subserving central retina (0 degrees-2 degrees) and in a region of representation of parafoveal retina (4 degrees-7 degrees) were studied in unanesthetized paralyzed macaque monkeys. Neurons sensitive to the orientation of the stimulus in the visual field (simple, complex, and hypercomplex), and neurons lacking orientation selectivity (concentric, and a new class termed uniform) were found. In foveal cortex non-oriented cells were more numerous, and orientation sensitive cells had less strict spatial stimulus requirements than in parafoveal cortex. Most neurons received a monocular input, either exclusively or very predominantly. Three types of neurons were recognized on the basis of their responses to chromatic stimuli. (1) Luminosity neurons (about half the population) gave the same qualitative response to all effective wayelengths and had a spectral sensitivity similar to that of the macaque, determined behaviorally. Cells with all spatial types of receptive fields, except simple, occurred in this group. (2) Spectrally-treated neurons also responded in the same manner to different wavelengths, but over a narrower range than luminosity neurons, and their maximal sensitivity was shifted toward one or the other end of the visible spectrum. All tuned neurons had uniform or complex receptive field. (3) Spectrally-opponent neurons were either excited or inhibited by long wavelengths and responded in the opposite manner to short wavelengths. For cells with uniform or complex receptive fields the two opponent systems were coextensive. Simple or concentric neurons often had dual-opponent organization. The distribution of functional types among different cortical layers was similar in parafoveal and foveal cortex. The functional attributes of ocular dominance and orientation sensitivity were found to be statistically independent dimensions of cortical organization. On the other hand, the correlation between spatial and chromatic properties did not vary between different cytoarchitectonic layers, a finding suggesting that these neuronal properties depend on conjoined projectional and intracortical connecting mechanisms.

Fixation behavior while walking: persons with central visual field loss.

The aim of this study was to determine the effect of central visual field loss (CFL) on fixation patterns of a person walking towards a target. Subjects were four visually normal persons and 10 persons with CFL. Eye position on scene was recorded and classified into 20 scene categories. The distributions of fixations among scene categories were compared across the two subject groups. For all but two CFL subjects, who fixated primarily at the floor, the distributions of fixations for the CFL subjects ranged from being moderately to strongly correlated with that of the visually normal mean. An analysis of the similarity in the sequence of fixations (or gaze pattern) of the CFL subjects to the visually normal subjects showed a range of 7-66%. Excluding the one CFL subject who had a functioning fovea, sequence similarity was strongly correlated with the logarithm of the minimum angle of resolution (logMAR). The better a persons logMAR, the more closely his or her gaze pattern matched that of the visually normal subjects. Finally, the CFL data were tested against two current models of oculomotor strategy, visual salience and guided search. Similar to what was found with visually normal subjects, CFL subjects appear to use the expected features and general location of the target to guide their fixations, the guided-search strategy.

Extracellular Recordings from Human Retinal Ganglion Cells

Ganglion cells were studied in the isolated retina, with extracellular recordings. Activity was found similar to that seen in the retinas of other animal species.

A method for projecting retinal landmarks into visual space.

In acute visual neurophysiology experiments on paralyzed animals, it is frequently necessary or desirable to specify receptive field positions in terms of coordinates centered on the axis of gaze. For experiments utilizing a tangent screen this requires a projection, or mapping of the fovea or area centralis onto the screen in order to establish the origin of the coordinate system. The mapping has been done with a specially constructed ophthalmoscope (Hubel and Wiesel, l%O>. In principle this device throws a narrow beam through the center of the pupil and is positioned so that the beam falls on the retinal point which is to be projected. A second beam exits the ophthalmoscope at an angle of 180” from the first beam along the same axis, thereby projecting along the visual direction of the retinal point. In a variation of this method (Bishop, Kozak and Vakkur, 1962) the ophthalmoscope has only one beam, which is rotated 180” after the retinal point has been sighted. Femald and Chase (1971) have described a way of performing this projection directly in cats. The fundus is brightly illuminated and the reflected rays are focused by the optics of the eye and auxiliary correction lenses onto the tangent screen. Unfortunately, we have not found this a useful technique in the monkey, whose smaller optical aperture limits the available light from the eye too severely to allow visualization of a projected image. We have devised a method which is useful in the monkey and which allows rapid and very precise determinations, provided that the retinal landmark to be projected can be well visualized ophthalmostopically. A photograph of this instrument is shown in Fig. 1 and a schematic diagram of the optics is given in Fig. 2. Light from a small source S passes through two lenses, LZ and L,, and is reflected off mirror M into the eye of the animal. The direction taken by the central ray of the optical system after reflection, designated R in the schematic, will be determined by the angular position of mirror M. Micrometer AZ controls the rotation of the mirror around a roughly vertical axis by pushing a connecting arm attached to the shaft supporting the mirror. The mirror support shaft pivots on a bracket which attaches to the block on which microme-