Jennifer S. Lund

A model for the intracortical origin of orientation preference and tuning in macaque striate cortex

We report results of numerical simulations for a model of generation of orientation selectivity in macaque striate cortex. In contrast to previous models, where the initial orientation bias is generated by convergent geniculate input to simple cells and subsequently sharpened by lateral circuits, our approach is based on anisotropic intracortical excitatory connections which provide both the initial orientation bias and its subsequent amplification. Our study shows that the emerging response properties are similar to the response properties that are observed experimentally, hence the hypothesis of an intracortical generation of orientation bias is a sensible alternative to the notion of an afferent bias by convergent geniculocortical projection patterns. In contrast to models based on an afferent orientation bias, however, the intracortical hypothesis predicts that orientation tuning gradually evolves from an initially nonoriented response and a complete loss of orientation tuning when the recurrent excitation is blocked, but new experiments must be designed to unambiguously decide between both hypotheses.

The visual system of the mole. Talpa europaea.

Abstract Moles were trained to discriminate between light and dark stimuli in a modified Yerkes box. Four learned the problem to a criterion of 90% correct responses over thirty trials, the threshold being at the level of 2.9 to 3.0 log foot lamberts in three of them. They generally showed no strong preference for the light or dark sides when first tested in the apparatus; but all showed a withdrawal response to flashing light. The central visual pathways of the mole were studied in two of these animals and in four others by the Nauta method after unilateral eye enucleation. The pretectum was found to be the principal primary optic center although considerable variation in the distribution of optic fibers was noted in different animals. There was a good supply to the dorsal lateral geniculate body in only one animal studied; but there was a suggestion of preterminal degeneration in the ventral part of the nucleus in all animals. Degeneration was consistently absent in the superior colliculus; and a well-defined accessory optic tract could not be traced.

Visual Control of Limb Movement Following Section of Optic Chiasm and Corpus Callosum in the Monkey

Summary Hand preference tests were given to four monkeys following section of the optic chiasm and anterior commissure and also after additional section of the corpus callosum. Using a stationary food reward and testing under conditions of binocular and monocular vision, no change was found, in hand preference with changes in visual input following chiasmal section. Marked preference for using the hand contralateral to the seeing eye was shown following additional section of the corpus callosum with enforced monocular vision, and abnormalities in the use of the hand ipsilateral to the open eye were evident. These results indicate that deficits in visuomotor control occur in the “split-brain” monkey independently from visual pattern learning situations and suggest a direct cortical link between visual input and crossed corticospinal fibers, giving dexterity to finger movements. A comparison of the speed of retrieval of a moving target was made between normal monkeys, those with optic chiasm sectioned and those with optic chiasm, corpus callosum and anterior commissure sectioned. With binocular vision, only the “split-brain” animals differed from normal, being slower and more erratic in their performance. With monocular testing the animals with only the optic chiasm sectioned showed a slight initial deficit, attributed to the adjustment to their visual field deficit. The “split-brain” animals showed marked disturbance of performance of the tasks. On occluding the first eye by lid suture and testing over a period of one to several weeks, their two hands performed initially at the same rate, then a marked deficit of the hand ipsilateral to the open eye became evident, then again improved. On changing directly to occlusion of the second eye a marked deficit or refusal to perform the task was shown using either forelimb; then over time a gradual improvement was evident, particularly with the forelimb contralateral to the open eye. These findings lead to a theory of gradual bias of subcortical visuomotor linkages under the influence of lateralised vision in the absence of the corpus callosum. The repeated use of new visuomotor linkages disrupts unused, previously established, linkages. The utilisation of intrahemispheric visuomotor links can delay or disrupt the simultaneous formation of the apparently weaker interhemispheric linkages in the “split-brain” animal.

Adaptation to visual field displacement by monkeys with optic chiass sectioned

Abstract Visuomotor skill in the monkey has been examined extensively by previous investigators following section of the optic chiasm and forebrain commissures. The effect of section of the optic chiasm alone on visual guidance of the limbs has, however, received little attention. In the present study, midline section of the optic chiasm and enforced monocular vision in the monkey was found to result in a shift in position of errors made by the animals in reaching for a target, as compared to normal animals. The errors made occurred more to one side of the target than the other, being more frequently made on the side of the intact half visual field. This tendency for errors in reach to occur to one side of the target under conditions of monocular vision, in the animals with chiasm sectioned, had a marked influence on adaptation of arm movement in compensation for a monocularly worn prism. These results showed an apparent lack of interocular transfer of adaptation to the prism (i.e., no demonstrable aftereffect of the training with the prism using the trained hand and the eye which had been occluded during adaptation to the prism). A consideration of this asymmetric error of visually guided movement produced by optic chiasm section and its effect on adaptation to lateral deviation of the visual field has been made, since the lack of interocular transfer might imply the presence of a purely visual component, independent of proprioception, to the process of guidance of limb movements. The finding of abnormal visual guidance of the limbs following optic chiasm section alone is also important in the assessment of defects in visuomotor skills in split-brain animals.

Blind Signal Separation from Optical Imaging Data

Optical imaging is the video recording of two-dimensional patterns of changes in light reflectance from cortical tissue evoked by Stimulation. We derived a method, called extended spatial decorrelation (ESD), that uses second order statistics in space for separating the intrinsic signals into the stimulus related components and the nonspecific variations. The Performance of ESD on model data is compared to independent component analysis (ICA) algorithms using statistics of 4th and higher order. Robustness against sensor noise is scored. When applied to optical images, ESD separates the stimulus specific signal well from biological noise and artifacts.

Influence of Recurrent Excitation and Inhibition on Receptive Field Size and Contrast Sensitivity in Layer 4C of Macaque Striate Cortex

Neurons in layer 4C in macaque striate cortex show an increase in receptive field size and achromatic contrast sensitivity from the bottom to the top of the layer. Using a computational model which is based on realistic anatomical and physiological data we demonstrate that part of the observed changes can arise from differences in the overall balance between recurrent excitation and lateral inhibition from two different neuron types. The model predicts that — given the above hypotheses — lateral recurrent excitation must come from an increasingly wider range with rise in depth of layer 4C, and lateral inhibition must have higher threshold and gain in upper 4Cα. The anatomical substrate of recurrent excitation are the stepped projections of spiny stellate cells. As the possible anatomical substrate of differential inhibition we suggest the “clutch” cell in lower and mid 4C and the α-6 [1] cell in upper 4Cα which replaces the clutch cell as a somatic inhibitor.