Alan Cowey

Transcranial magnetic stimulation and cognitive neuroscience

Transcranial magnetic stimulation has been used to investigate almost all areas of cognitive neuroscience. This article discusses the most important (and least understood) considerations regarding the use of transcranial magnetic stimulation for cognitive neuroscience and outlines advances in the use of this technique for the replication and extension of findings from neuropsychology. We also take a more speculative look forward to the emerging development of strategies for combining transcranial magnetic stimulation with other brain imaging technologies and methods in the cognitive neurosciences.

The ganglion cell and cone distributions in the monkey's retina: implications for central magnification factors.

The distribution of cones and ganglion cells was determined in whole-mounted monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being up to three times greater in nasal retina. A similar but smaller asymmetry occurred with cones. The total number of ganglion cells varied from 1.4 to 1.8 X 10(6), agreeing well with counts of optic nerve axons. The variation of ganglion cell density with eccentricity indicates the magnification factor (MF) of the retina. This was compared with MF at the dorsal lateral geniculate nucleus and at striate cortex, revealing that the relative representation of the fovea increases substantially in both thalamus and cortex.

Human cortical magnification factor and its relation to visual acuity.

SummaryThe magnification factor (M) of the retina is the linear extent of visual striate cortex to which each degree of the retina projects. It has been suggested that magnification factor is directly proportional to visual acuity, but magnification factor measured in monkeys was compared with visual acuity in man. Here we first describe calculation of the magnification factor in man, and then compare it to human visual acuity.We calculated M for the first 30 degrees of the lower visual field by using infor mation provided by Brindley and Lewin (1968), who plotted the distribution of phosphenes evoked by stimulation of visual cortex in a human patient with electrodes implanted on the visual cortex. Since the inter-electrode distance was specified it was possible to calculate M for each of many pairs of electrodes by measuring the angular separation and mean eccentricity of the corresponding pairs of phosphenes. For the lower visual field, M was approximately 4 mm/degree at 2 degrees eccentricity and declined monotonically to 0.5 mm/degree at 25 degrees eccentricity.The results indicated that the reciprocal of M is directly proportional to the minimum angle of resolution and, correspondingly, that the magnification factor is directly proportional to visual acuity in man.By extrapolating this function for the whole of the visual field it was possible to estimate the area of striate cortex. The total extent of striate cortex estimated in this way agreed closely with previous direct measurements, suggesting that the measurements of M are accurate.

The axo-axonic interneuron in the cerebral cortex of the rat, cat and monkey

The synaptic connections of a specific type of identified cortical interneuron, the axo-axonic cell, were studied using Golgi methods. In the light-microscope axo-axonic cells were demonstrated in certain layers of the primary and secondary visual cortex of rat, cat and monkey, in the motor cortex of cat and in the subiculum and pyriform cortex of rat. The dendrites originating from the oval soma were oriented radially in a lower and upper spray within a cylinder about 100-150 microns wide. Electron-microscopy of Golgi impregnated, gold-toned axo-axonic cells showed predominantly but not exclusively asymmetrical synaptic contacts on their dendrites and spines, few synaptic contacts on the perikarya some of which were asymmetrical, and no synaptic contacts on the axon initial segment. The axon usually arborized within the vicinity of the cells own dendritic field in an area 100-200 microns in diameter. In the kitten motor cortex the axon of a neuron in layer III descended to layer VI, providing a columnar arborization. The axon formed specialized, 10-50 microns long terminal segments invariably oriented parallel with the axon initial segment of pyramidal cells. All 85 identified symmetrical-type synaptic contacts, deriving from 31 specialized terminal segments, were found exclusively on the axon initial segment of pyramidal neurons. Rare, lone boutons of axo-axonic cells also made synaptic contact only with axon initial segments, confirming the exclusive target specificity of these cells. In identified gold-toned boutons, flattened pleomorphic vesicles were present. Electron-microscopy showed that axons ending in specialized terminal segments may originate from myelinated fibres, indicating that Golgi impregnation has revealed only part of the axon. Counting of axon terminal segments, each of which was in contact with the axon initial segment of a pyramidal neuron, revealed 166 pyramidal neurons receiving input from a partially reconstructed axo-axonic cell in the motor cortex of the kitten, and 67 from another cell in the visual cortex of the cat. The convergence of five axo-axonic cells onto one pyramidal cell was demonstrated in the striate cortex of the cat by counting all synaptic contacts on three initial segments. Cells from a one-month-old kitten were compared with those of the adult. The axon of the developing neurons was more diverse, having many growth cones and filopodia which made no specialized membrane contacts. However, the developing specific terminal segments formed synapses only with axon initial segments. It is concluded that the presence of axo-axonic cells in all the species and cortical areas we have examined suggests their association with the structural design of pyramidal cells, wherever the latter occur, and with their participation in the information processing of pyramidal cells. Axo-axonic cells are uniquely endowed with the means of simultaneously influencing the action potential at the site of origin in groups of pyramidal cells...

Post-decision wagering objectively measures awareness

The lack of an accepted measure of awareness has made claims that accurate decisions can be made without awareness controversial. Here we introduce a new objective measure of awareness, post-decision wagering. We show that participants fail to maximize cash earnings by wagering high following correct decisions in blindsight, the Iowa gambling task and an artificial grammar task. This demonstrates, without the uncertainties associated with the conventional subjective measures of awareness (verbal reports and confidence ratings), that the participants were not aware that their decisions were correct. Post-decision wagering may be used to study the neural correlates of consciousness.

Effects of foveal prestriate and inferotemporal lesions on visual discrimination by rhesus monkeys

SummaryAblation of inferotemporal cortex in monkeys impairs visual discrimination learning, and inferotemporal cortex receives visual information from striate cortex by way of the circumstriate belt. Yet most previous studies have failed to find any discrimination impairment after partial ablations of the circumstriate belt.In this experiment severe impairments in post-operative acquisition and retention of visual discrimination problems were found after lesions of “foveal prestriate cortex”, i.e. the portion of the circumstriate belt which receives a projection from the cortical representation of the fovea in striate cortex and which lies, largely buried, in the ventrolateral portion of prestriate cortex. Although foveal prestriate lesions produced a greater impairment on individual pattern discrimination tasks than inferotemporal lesions, the opposite was true of concurrent visual discrimination tasks in which several different pairs of discriminanda are presented in each testing session until the animal learns to discriminate every pair.The results are related to a two-stage model of discrimination learning and it is suggested that foveal prestriate lesions impair visual attention or perception, whereas inferotemporal lesions disturb the associative or mnemonic stage of visual discrimination learning.

Topography of the retina and striate cortex and its relationship to visual acuity in rhesus monkeys and squirrel monkeys

SummaryDensity of cones and ganglion cells was studied in horizontal sections of retina in the rhesus monkey (Macaca, mulatta) and the squirrel monkey (Saimiri sciureus). The lower angular density of cones in the fovea of Saimiri correlates with its visual acuity, which is poorer than that of Macaca (0.74 and 0.65 min of arc respectively). Cone density falls more steeply with angular eccentricity from the fovea in Saimiri, in accordance with its relatively poorer peripheral acuity. Comparable results were obtained with retinal ganglion cells, but the comparisons at the fovea itself are more difficult because of the lateral displacement of these elements in the foveal region.The cortical magnification of the visual field (that is, the number of mm of cortex per degree of visual field) is lower for both the foveal and parafoveal representations at the striate cortex in Saimiri. This was correlated with its poorer foveal and parafoveal acuity. It was shown that with increasing eccentricity from the fovea, the fall in the magnification of the visual field at the striate cortex is approximately proportional to the decrease in ganglion cell density at the retina. The results of this study, in which acuity and topography of the visual system are compared in two species of monkey, are consistent with the view that both retinal topography, and the cortical magnification of the visual field, are closely related to visual acuity.

Cortical maps and visual perception the grindley memorial lecture

The lecture was given at the meeting of the Experimental Psychology Society in Cambridge, July 1978.

Effects of dorsal and ventral hippocampal lesions on spontaneous alternation, learned alternation and probability learning in rats

Abstract In a series of 3 experiments it was shown that lesions restricted to the dorsal or ventral portions of the hippocampus in rats produced different effects on behaviour. Animals with dorsal lesions perseverated on a spontaneous alternation task with a long inter-trial interval whereas those with ventral lesions alternated with a long delay and perseverated at short delays. On a probability learning task rats with dorsal lesions were indistinguishable from controls, whereas those with ventral lesions were impaired. When tested on lever-alternation the rats with dorsal hippocampal lesions were significantly superior to controls and those with ventral lesions were significantly impaired. The results are discussed in relation to the roˆle of dorsal hippocampus in attention and habituation and the ventral hippocampus in the mediation of response sets or strategies.

The neurobiology of blindsight

Some patients can respond to visual stimuli presented within their clinically absolute visual field defects that have been caused by partial destruction of striate cortex. This puzzling phenomenon of looking, pointing, detecting and discriminating without seeing has been called blindsight, and has fascinated philosophers and neuroscientists alike as a spotlight on the nature of unconscious or covert awareness, and the means it provides of studying the visual information carried by pathways other than the major route through the striate cortex.