John D. Pettigrew

The neural mechanism of binocular depth discrimination

1. Binocularly driven units were investigated in the cats primary visual cortex.

Analysis of retinal correspondence by studying receptive fields of rinocular single units in cat striate cortex

SummaryThe concept of corresponding retinal points was examined in terms of the binocular receptive fields of neurons in Area 17 of the cerebral cortex of the cat. Only a proportion of the binocular receptive field pairs can be accurately superimposed at the one time in a given plane. The fields which are not corresponding are said to show receptive field disparity. The attempt has been made to establish, on a quantitative basis, the parameters of the receptive field disparities that occur within 5° of the visual axis. A new method was used for defining the zero (vertical) meridian. Very effective paralysis of the extraocular muscles was achieved and the very small residual eye movements that occurred were regularly monitored so that corrections could be applied to the plotted positions of the receptive field pairs. The distribution of the receptive field disparities about the position of maximal correspondence has a range of about ±1.2° (S.D. 0.6°) in both the horizontal and vertical directions for fields in the vicinity of the visual axis. Panums fusional area may represent the extent to which receptive fields in the one eye, all with the same visual direction, are linked to fellow members of a pair in the other eye over a range of receptive field disparities. A naso-temporal overlap of receptive fields occurs which is probably little if any more than can be accounted for on the basis of the disparity of receptive fields lying along the zero (vertical) meridian. When the extraocular muscles are paralyzed the eyes diverge and the binocular receptive field pairs are separated on the tangent screen. The distribution of the horizontal and vertical separations of the receptive field pairs have been examined.

Responses to moving slits by single units in cat striate cortex

SummaryA quantitative study has been made of the responses to moving slit stimuli by single units in the cat striate cortex whose receptive fields lay within 5° of the visual axis. Special attention was given to finding the optimal stimulus parameters including slit width, length, orientation and speed. The analysis was largely based on averaged response vs. time histograms. Using the classification of simple and complex responses types, the units were further subdivided on the basis of the number of modes in the response and on the presence or absence of directional selectivity. Simple unimodal units with directional selectivity (SUDS) had the most specific stimulus requirements and nearly always had zero background activity. Complex units usually had a high level of background activity. SUDS units also showed a preference for horizontally- and vertically ****-orientated stimuli. Whenever the response survived reversal of contrast the directional selectivity remained independent of the change. Optimal stimulus speeds varied widely from unit to unit with a mean at 4°/sec: simple bimodal units and complex units tended to have higher optimal stimulus speeds and responded over a wider range of speeds than did simple unimodal units. While SUDS units with very small receptive fields tended to prefer slowly moving stimuli, in general there was no correlation between receptive field size and optimal stimulus speed.

Binocular interaction on single units in cat striate cortex: simultaneous stimulation by single moving slit with receptive fields in correspondence.

SummaryAveraged responses of binocularly-activated single units of the striate cortex of paralysed cats were studied using a single, moving visual stimulus and prisms of variable power to control the visual direction of each eye. Binocular facilitation, summation or occlusion of the monocular response occurred, depending on the type of unit and on the prism setting. Binocular stimulating conditions were optimal for a given unit when the prism setting superimposed, or very nearly superimposed, the receptive field pair on the same plane as the moving stimulus. Under these optimal conditions, most units showed summation or facilitation of the monocular responses, with a minority showing occlusion. When the prism setting was changed from the optimal value, binocular occlusion could be demonstrated in all units.Curves plotting binocular response against prism setting provided information on the specificity, temporal properties and symmetry of the binocular response. The binocular response of simple units showed great specificity with a sharply defined peak on the response curve at a particular prism setting. There was variation from one simple unit to another in the exact prism setting required to give the optimal response. A proportion of complex units, despite large receptive field size, showed binocular specificity with a very narrow range of facilitation, of the same order as that shown by simple units. Other complex units showed binocular facilitation over a wide range of prism settings.

Local perfusion of noradrenaline maintains visual cortical plasticity

WE have formed a hypothesis which links two important, but so far separate, research areas, the monoaminergic system discovered by the Swedish group1,2 and the phenomenon of critical period plasticity in the visual cortex discovered by Wiesel and Hubel3,4. We propose5 that the widespread system of monoaminergic fibres plays a part in regulating plasticity and that, more specifically, catecholamines are responsible for maintaining the high level of plasticity which is observed in the visual cortex during the critical period4. In an initial test of this hypothesis, we developed a dose and timing regimen of 6-hydroxydopamine (6-OHDA) to produce significant depletion of catecholamines bilaterally in the visual cortex of developing kittens5. The hypothesis was confirmed to the extent that kittens treated with 6-OHDA do not have the usual cortical plasticity, as measured by a change in the ocular dominance of binocular neurones following monocular occlusion5. While all the results we have obtained so far with 6-OHDA are consistent with the view that catecholamines regulate cortical plasticity, other interpretations are possible because of the widespread nature of the changes accompanied by intraventricular 6-OHDA. We now present further evidence in support of the hypothesis from experiments involving microperfusion of catecholamine in localised areas of the visual cortex of animals which would not be expected to show plasticity. These experiments indicate a specific role of noradrenaline (NA) within the cortex because plastic changes are found only in the region of cortex perfused by NA while nearby cortical regions in the same kitten are unaffected.

Peak density and distribution of ganglion cells in the retinae of microchiropteran bats: implications for visual acuity.

We have estimated the total number, distribution and peak density of retinal ganglion cells (RGCs) in retinal wholemounts of several species of microchiropteran (echolocating) bats. The estimates are based on counts of Nissl-stained, presumed RGCs. The total number of presumed RGCs varies among the species: from about 4,500 in Rhinolophus rouxi to about 120,000 in Macroderma gigas. In addition, in two species (Nyctophilus gouldi and M. gigas), the estimates are based on counts of positively identified RGCs retrogradely labelled with the enzyme horseradish peroxidase injected into the retinorecipient nuclei. In these two species, the numbers and distributions of retrogradely labelled RGCs and Nissl-stained presumed RGCs are very similar. In all six species studied, the peak-density regions of presumed (or positively identified) RGCs are located in the inferotemporal retinae, and the RGC isodensity lines tend to be horizontally elongated. However, the RGC densities in the high-density regions are only 2-4 times greater than those in the low-density regions in the superior retinae. The somal sizes of RGCs vary from 5 to 16 micron in diameter and are unimodally distributed. There is no indication of the existence of distinct morphological classes of RGCs. The axial lengths of microchiropteran eyes vary from 1.8 mm in R. rouxi to 7.0 mm in M. gigas. For all species the posterior nodal distance (PND) was assumed to be 0.52 of the axial length of the eye. This assumption is based on the analysis of published data concerning schematic eyes of nocturnal vertebrates. These derived values of the PNDs allowed us to calculate the retinal magnification factors and the number of RGCs per degree of visual angle. From these, the upper limits of visual acuity were derived on the basis of the assumptions of the sampling theorem. The estimated upper limits of visual acuity of the six species of echolocating bats vary from about 0.35 cycles/degree in R. rouxi to about 2 cycles/degree in M. gigas. This range is quite similar to the range of visual acuities in murid rodents.

Meditation alters perceptual rivalry in Tibetan Buddhist monks

Neuroscientific studies of the mind are likely to benefit from the insights and skills of Tibetan Buddhist monks who have practiced the historic tradition of meditative training over many years — a point made recently at a forum between a selection of Buddhist leaders and distinguished scientists. Perceptual rivalries, such as binocular rivalry and motion induced blindness, are being used to study the neural mechanisms underlying consciousness and attention, as they involve fluctuations in conscious awareness despite unchanging external stimulation. Tapping into the ability of Tibetan Buddhist monks to control the flow of items being attended to and accessing consciousness, we found that meditation alters the inherent fluctuations in conscious state associated with perceptual rivalry.

Retinal topography in reef teleosts. II: Some species with prominent horizontal streaks and high-density areae

The retinal ganglion cell layer of five species of reef teleosts was studied from Nissl-stained whole-mounts and the distribution of neural elements determined quantitatively. Iso-density contour maps of neurons in the ganglion cell layer revealed a temporal area centralis (ranging from 3.5 to 8.3 x 10(4) cells/mm2) which often extended into a horizontal streak (ranging from 1.4 to 5.0 x 10(4) cells/mm2) across the retinal meridian. Species possessing a marked horizontal streak were found to inhabit open water and perceive their environment with an uninterrupted view of sand-water horizon. The behavioural significance of these horizontal areas of acute vision is also discussed.

A ‘sticky’ interhemispheric switch in bipolar disorder?

Despite years of research into bipolar disorder (manic depression), its underlying pathophysiology remains elusive. It is widely acknowledged that the disorder is strongly heritable, but the genetics are complex with less than full concordance in monozygotic twins and at least four susceptibility loci identified. We propose that bipolar disorder is the result of a genetic propensity for slow interhemispheric switching mechanisms that become ‘stuck’ in one or the other state. Because slow switches are also ‘sticky’ when compared with fast switches, the clinical manifestations of bipolar disorder may be explained by hemispheric activation being ‘stuck’ on the left (mania) or on the right (depression). Support for this ‘sticky’ interhemispheric switching hypothesis stems from our recent observation that the rate of perceptual alternation in binocular rivalry is slow in euthymic subjects with bipolar disorder (n=18, median = 0.27Hz) compared with normal controls (n=49, median = 0.60Hz, p0.0005). We have presented evidence elsewhere that binocular rivalry is itself an interhemispheric switching phenomenon. The rivalry alternation rate (putative interhemispheric switch rate) is robust in a given individual, with a test–retest correlation of more than 0.8, making it suitable for genetic studies. The interhemispheric switch rate may provide a trait-dependent biological marker for bipolar disorder.

A common oscillator for perceptual rivalries

Perceptual rivalry is an oscillation of conscious experience that takes place despite univarying, if ambiguous, sensory input. Much current interest is focused on the controversy over the neural site of binocular rivalry, a variety of perceptual rivalry for which a number of different cortical regions have been implicated. Debate continues over the relative role of higher levels of processing compared with primary visual cortex and the suggestion that different forms of rivalry involve different cortical areas. Here we show that the temporal pattern of disappearance and reappearance in motion-induced blindness (MIB) (Bonneh et al, 2001 Nature 411 798–801) is highly correlated with the pattern of oscillation reported during binocular rivalry in the same individual. This correlation holds over a wide range of inter-individual variation. Temporal similarity in the two phenomena was strikingly confirmed by the effects of the hallucinogen LSD, which produced the same, extraordinary, pattern of increased rhythmicity in both kinds of perceptual oscillation. Furthermore, MIB demonstrates the two properties previously considered characteristic of binocular rivalry. Namely the distribution of dominance periods can be approximated by a gamma distribution and, in line with Levelts second proposition of binocular rivalry, predominance of one perceptual phase can be increased through a reduction in the predominance time of the opposing phase. We conclude that (i) MIB is a form of perceptual rivalry, and (ii) there may be a common oscillator responsible for timing aspects of all forms of perceptual rivalry.