Auditory cortical contrast enhancing by global winner-take-all inhibitory interactions.

Brains decompose the world into discrete objects of perception, thereby facing the problem of how to segregate and selectively address similar objects that are concurrently present in a scene. Theoretical models propose that this could be achieved by neuronal implementations of so-called winner-take-all algorithms where neuronal representations of objects or object features interact in a competitive manner. Here we present evidence for the existence of such a mechanism in an animal species. We present electrophysiological, neuropharmacological and neuroanatomical data which suggest a novel view of the role of GABA(A)-mediated inhibition in primary auditory cortex (AI), where intracortical GABA(A)-mediated inhibition operates on a global scale within a circular map of sound periodicity representation in AI, with functionally inhibitory projections of similar effect from any location throughout the whole map. These interactions could underlie the proposed competitive "winner-take-all" algorithm to support object segregation, e.g., segregation of different speakers in cocktail-party situations.

Differential effects of iontophoretic in vivo application of the GABA(A)-antagonists bicuculline and gabazine in sensory cortex.

We have compared the effects of microiontophoretic application of the GABA(A)-receptor antagonists bicuculline (BIC) and gabazine (SR95531) on responses to pure tones and to sinusoidally amplitude-modulated (AM) tones in cells recorded extracellularly from primary auditory cortex (AI) of Mongolian gerbils. Besides similar effects in increasing spontaneous and stimulus-evoked activity and their duration, both drugs elicited differential effects on spectral tuning and synchronized responses to AM tones. In contrast to gabazine, iontophoresis of the less potent GABA(A)-antagonist BIC often resulted in substantial broadening of frequency tuning for pure tones and an elimination of synchronized responses to AM tones, particularly with high ejecting currents. BIC-induced effects which could not be replicated by application of gabazine were presumably due to the well-documented, non-GABAergic side-effects of BIC on calcium-dependent potassium channels. Our results thus provide strong evidence that GABA(A)-mediated inhibition in AI does not sharpen frequency tuning for pure tones, but rather contributes to the processing of fast temporal modulations of sound envelopes. They also demonstrate that BIC can have effects on neuronal response selectivity which are not due to blockade of GABAergic inhibition. The results have profound implications for microiontophoretic studies of the role of intracortical inhibition in sensory cortex.

Optical imaging of orientation and ocular dominance maps in area 17 of cats with convergent strabismus.

Strabismus (or squint) is both a well-established model for developmental plasticity of the brain and a frequent clinical symptom. While the layout and topographic relationship of functional domains in area 17 of divergently squinting cats has been analyzed extensively in recent years (e.g. Löwel et al., 1998), functional maps in convergently squinting animals have so far not been visualized with comparable detail. We have therefore investigated the functional organization of area 17 in adult cats with a surgically induced convergent squint angle. In these animals, visual acuity was determined by both behavioral tests and recordings of visual evoked potentials, and animals with comparable acuities in both eyes were selected for further experiments. The functional layout of area 17 was visualized using optical imaging of intrinsic signals. Monocular iso-orientation domains had a patchy appearance and their layout was different for left and right eye stimulation, so that segregated ocular dominance domains could be visualized. Iso-orientation domains exhibited a pinwheel-like organization, as previously described for normal and divergently squinting cats. Mean pinwheel density was the same in the experimental and control animals (3.4 pinwheel centers per mm2 cortical surface), but significantly (P < 0.00001) higher than that reported previously for normal and divergently squinting cats (2.7/mm2). A comparison of orientation with ocular dominance maps revealed that iso-orientation domains were continuous across the borders of ocular dominance domains and tended to intersect these borders at steep angles. However, in contrast to previous reports in normally raised cats, orientation pinwheel centers showed no consistent topographical relationship to the peaks of ocular dominance domains. Taken together, these observations indicate an overall similarity between the functional layout of orientation and ocular dominance maps in area 17 of convergently and divergently squinting cats. The higher pinwheel densities compared with previous reports suggest that animals from different gene pools might generally differ in this parameter and therefore also in the space constants of their cortical orientation maps.

GABA-inactivation attenuates colinear facilitation in cat primary visual cortex.

Neurons in primary visual cortex (V1) respond preferentially to stimuli of a particular orientation falling within a circumscribed region of visual space known as their receptive field (RF). However, the response to an optimally oriented stimulus presented within the RF can be enhanced by the simultaneous presentation of co-oriented, co-linearly aligned flank stimuli falling outside the RF which, when presented alone, fail to activate the cell. This type of contextual effect, termed colinear facilitation, presumably forms the physiological substrate for the integration of the line elements of a contour and the perceptual saliency of a contour in a complex environment. Here we show that colinear facilitation in single cells of cat area V1 can be substantially reduced or abolished by focal inactivation of laterally remote cells in the same area which respond strongly to the co-oriented, colinear flank stimulus inducing the facilitatory effect. The results provide evidence that horizontal intrinsic connections between cells with co-oriented and co-linearly aligned RFs make a major contribution to colinear facilitation in V1. They imply that the neuronal circuitry underlying contour integration and saliency is already present at the earliest stage of visual cortical information processing.

Neurophysiological Correlates of Colour Induction on White Surfaces.

Coloured light surrounding a white surface of about equal luminance makes the white surface appear illuminated with an unsaturated light of the complementary colour. In an attempt to discover the neurophysiological basis of such colour induction, we recorded from spectrally opponent cells of the parvocellular layers of the lateral geniculate nucleus (P-LGN) of anaesthetized macaques. Only cells with wide-band (W) spectral sensitivity in the short (S) or long wavelength (L) part of the spectrum (WS, WL) are excited by white spots of light centred on their receptive field. Cells with narrow-band (N) spectral sensitivity (NS, NL) and light-inhibited (LI) cells are inhibited by white light. Therefore, it is likely that the code for white is contained in a balanced excitation of the W cells. The effects of continuous illumination of remote surrounds with different wavelengths on the responses to achromatic light stimuli were investigated. Responses [on minus maintained discharge rate (MDR) or on-minus-off] were determined for white spots (1 - 3 degrees diameter) flashed on the receptive field centre, presented either alone or in the presence of an annular surround of equal luminance (inner diameter 5 degrees; outer diameter 20 degrees ). During red surround illumination the responses of WL cells to white spots tended to be reduced as were those of WS cells during blue surround illumination. Surround illumination with the opponent colour had more variable effects, neither WS nor WL cells showing a significant alteration of their mean response to white during surround illumination with opponent light. Response alterations were to a large extent due to changes in MDR, which increased in WS cells during blue surround illumination and in WL cells during red surround illumination. It is argued that the surround effects on centre responses are due to intraocular stray light rather than lateral connections in the retina. The surround effects also depended to some extent on the size of the test spot. LI cells and the very rare parvocellular panchromatic on-cells showed no chromatic response changes during coloured surround illumination. Inasmuch as the excitation of WS cells, either alone or in combination with NS cell activation, is involved in coding for green and blue, and that of WL cells, in combination with NL cell activation, is involved in coding for red and yellow in perception, the shift of excitation towards one or the other W cell group indicates relatively more red or green signals in the white response, consistent with and in the same direction as colour induction. In addition, the summed population response of WS and WL cells is decreased during surround illumination with any colour including white. This is related to brightness decrease during surround illumination in perception.