If physiological mechanisms similar to cat Y and X cells explain faster detection of low spatial frequencies by humans, then cats should show the same effect. We have tested this prediction by determining the visual reaction time of cats over a range of spatial frequencies and contrasts by training them to respond quickly when a vertical sine-wave grating was presented. At 50% contrast, the cat's visual reaction time increased monotonically from 0.25–2.0 cpd (cycle/deg). At every spatial frequency tested, the cat's reaction time increased monotonically as contrast decreased. By determining contrast threshold (70% detection) at each spatial frequency, it was possible to determine reaction times for different spatial frequencies at equal physical contrasts and equal “threshold equivalent” contrasts. Some of the cat's faster detection of low spatial frequencies was due to sensitivity differences and some was not. To determine if faster detection of low spatial frequencies was based upon Y cells, we took advantage of the fact that Y cells show a strong peripheral effect while X cells do not. Low and high spatial frequencies were detected in the presence of a flickering (7 Hz) or steady (70 Hz) surround. Surround frequency had no effect upon reaction times to 2.0 cpd but the flickering surround increased reaction times to 0.25 cpd. These results indicate that, in cats, rapid detection of low spatial frequencies is by Y cells and slower detection of high spatial frequencies is by X cells.

X- and Y-mediated input to areas 17 and 18 of the cat visual cortex was studied using current-source-density analysis of field potentials evoked by stimulation of the optic nerves. A cuff-shaped electrode was used for stimulation so that Y axons, by virtue of their larger diameters, would have lower electrical thresholds than X axons. The effect in each cortical area of activating Y axons alone could therefore be determined by low-&litude stimulation of the optic nerves. Current-source densities were calculated by two separate methods. (1) In five experiments, field potentials were measured sequentially at different cortical depths with a single tungsten electrode. Current densities were then calculated by computer. (2) In two experiments, current densities were derived in real time from field potentials recorded simultaneously from three sites with a multi-electrode probe. The calculation was performed by an analog circuit specially designed for this purpose. This method has several advantages over the standard, single-electrode method. At stimulus strengths sufficient to activate the majority of Y axons in the optic nerves, but subthreshold to most X axons, the field potentials evoked in area 17 changed little from layer to layer. When the current-source-density analysis was applied to these potentials, no significant sources or sinks were detectable. Only when the stimulus strength was raised to the point that both X and Y axons were activated by the stimulus were any current sources or sinks detected in area 17. The currents were similar in time course and laminar pattern to those recorded after stimulation of the optic chiasm. In area 18, large sources and sinks were evoked by stimulation of Y axons alone. These currents changed little when the stimulus strength was increased to activate X axons as well. Area 18, therefore, in contrast to area 17, seems to be dominated by Y input and receives little X input. These results support the conclusions of the accompanying paper in which synaptic potentials were recorded intracellularly from cortical neutrons. The intracellular experiments failed to show substantial Y input to area 17. The projections of X and Y axons may therefore be much more highly segregated into areas 17 and 18 than previously thought. Alternatively, the nature of the Y input to area 17 may be very different from that to area 18 in that it cannot be easily detected with intracellular or current-source-density techniques.

Cat-301 is a monoclonal antibody which recognizes a cell surface associated antigen of selected neurons in the central nervous system (CNS). In the visual system, cat-301 selectively labels Y-like cells in several visual structures, including portions of the lateral geniculate nucleus complex and visual cortex. The cat superior colliculus (SC) also receives Y input and contains cells driven by Y input which are selectively distributed in the deep superficial gray and deeper laminae. If cat-301 is selective to the Y-cell system in SC, labeled cells should be restricted to those laminae. To test this hypothesis, we have examined quantitatively the laminar distribution, percentage, size, and morphology of cells in SC labeled by the cat-301 antibody. Cat-301 labeled a variety of cells in the cat SC. Labeled cells were found within the deep portion of the superficial gray layer (6.6%), optic layer (27.6%), intermediate gray layer (26.9%), and the deep gray and white layers (38.5%). By contrast, only 2 of 667 labeled cells (0.3%) were found within that part of the upper superficial gray layer innervated exclusively by W input and thought to contain only W-driven cells. When considered as a percentage of the total cell population, cat-301 labeled cells represented less than 3% of cells in the superficial gray layer and approximately 15% in the deeper layers. Neurons labeled by cat-301 were all of medium to large size (mean average diameter = 33.3μm; range = 15–84μm) and included vertical fusiform and stellate cells in the upper layers and the very large neurons found in the intermediate gray and deeper layers. These results provide further evidence that the cat-301 antibody selectively recognizes the Y channel of the cat visual system.

We report on the existence of lagged Y (YL) cells in the A laminae of the cat lateral geniculate nucleus (LGN) and on criteria for identifying them using visual and electrical stimulation. Like the lagged X (XL) cells described previously (Mastronarde, 1987a; Humphrey & Weller, 1988a), YL cells responded to a spot stimulus with an initial dip in firing and a delayed latency to discharge after spot onset, and an anomalously prolonged firing after spot offset. However, the cells received excitatory input from retinal Y rather than X afferents, and showed nonlinear spatial summation and other Y-like receptive-field properties. Three YL cells tested for antidromic activation from visual cortex were found to be relay cells, with long conduction latencies similar to those of XL cells.

Simultaneous recordings of a YL cell and its retinal Y afferents show striking parallels between lagged X and Y cells in retinogeniculate functional connectivity, and suggest that the YL-cell response profile reflects inhibitory processes occurring within the LGN. The YL cells comprised -5% of Y cells and -1% of all cells in the A laminae. Although infrequently encountered in the LGN, they may be roughly as numerous as Y cells in the retina, and hence could fulfill an important role in vision.

When a cuff-shaped electrode is placed on the optic nerve of the cat, X and Y axons, by virtue of their different diameters, exhibit different thresholds to electrical stimulation. Large-diameter Y axons have low thresholds, while smaller-diameter X axons have high thresholds. There is very little overlap between the two populations. Given this segregation, the strength of stimulation of the optic nerve required to evoke synaptic potentials in cortical neurons becomes a reliable indicator of the type of visual input a cortical neuron receives. Potentials with thresholds below the thresholds of X axons must be mediated by Y cells of the retina and LGN. Potentials with thresholds above the Y axons of the optic nerve must be mediated by X cells. From previous experiments, one would expect ot find &le input via both types of axon to are 17 of the visual cortex. This was not the case. Of 58 neurons distributed throughout the layers of area 17 from which intracellular records were taken, in only four could substantial Y excitation be detected. Three of these four were located near the border with area 18. All four received large X inputs as well. The 24 neurons studied in area 18 all received large Y inputs but no detectable X input.