Influence of visual cortex on light impulses in the brain stem of the unanesthetized cat

Visibility of brief images: the dual-process approach

If successive, brief visual images are exposed for recognition or for psychophysical ratings, various effects and phenomena of fast dynamics of conscious perception such as mutual masking, metacontrast, proactive enhancement of contrast, proactive speed-up of the latency of subjective visual experience, the Fröhlich Effect, the Tandem Effect, attentional facilitation by visuospatial precuing, and some others have been found. The theory proposed to deal with these phenomena proceeds from the assumption that two types of brain processes are necessary in order to consciously recognize visual stimuli: (1) fast, specific processes of encoding that allocate and reactivate the stimulus representation which is based on the activity of selected cortical neurons and (2) relatively slower processes of facilitation of the activity of this specific representation that are mediated by the excitatory modulation of the EPSPs of those selected cortical neurons by the ascending input from nonspecific thalamus. The perceptual retouch construct is proposed in order to characterize and analyze the interaction of (1) and (2). The neurophysiological characteristics of this bifunctional system of afference help to put forward several predictions that are found to be consistent with the empirical regularities of the above-described perceptual-attentional phenomena. These data form a body of converging evidence that is consistent with the predictions of the perceptual retouch approach.

The central release of acetylcholine during stimulation of the visual pathway

1. In rabbits anaesthetized with Dial ACh has been collected from the surface of the cerebral cortex during stimulation of the visual pathways.2. The spontaneous release of ACh from the visual and non-visual areas of the cortex was found to be similar.3. Stimulation of the retinae by diffuse light produced a large increase in ACh release from the primary visual receiving areas (4.3 times the spontaneous release) and a smaller increase (1.9 times the spontaneous release) from other parts of the cortex.4. Direct unilateral electrical stimulation of the lateral geniculate body evoked a large increase in ACh release (3.4 times the spontaneous release) from the ipsilateral visual cortex and a smaller increase (1.7 times the spontaneous release) from the contralateral visual area and other regions of the cerebral cortex. The evoked increase from the contralateral cortex was not mediated by transcallosal pathways.5. The increase in ACh release evoked from the visual cortex by stimulation of the ipsilateral lateral geniculate body was dependent on the frequency of stimulation. The evoked release was smallest at low stimulus frequencies and increased to a maximum at 20 stimuli/sec. The evoked ACh release from other areas of the cortex was independent of the frequency at which the lateral geniculate body was stimulated.6. The possible central nervous pathways associated with the spontaneous release of ACh and the release evoked by stimulation of the eyes by light and by direct stimulation of the lateral geniculate body are discussed.7. It is concluded that two ascending cholinergic systems may be involved; the non-specific reticulo-cortical pathways responsible for the e.e.g arousal response, and the more specific thalamo-cortical pathways associated with augmenting and repetitive after-discharge responses. The first system is thought to be concerned with the small but widespread increase in ACh release from the cortex following stimulation of the visual pathway while the second system could give rise to the larger increases evoked from the primary receiving areas of cortex. The spontaneous release of ACh from the surface of the brain may be the result of contributions from both systems.