The Effects of Attentional Spread and Attentional Effort on Orientation Discrimination

We investigated the spatial properties of visual attention, and its relation to attentional effort. We show that stimulus detectability changes as a function of attentional beam width and of the degree of task difficulty. We used a matching-to-sample paradigm, presenting two Gabor patches simultaneously as sample stimuli and as test stimulus, the stimuli being at three different distances. Task difficulty was graded by changing the orientation difference of the two Gabor patches on nonmatching trials. ‘Difficult’ nonmatching probe trials were embedded within an easy block of trials (easy condition), and vice versa for ‘easy’ probe trials. Differences in the detectability, d', between probe trials in the two conditions were taken as a measure for the change in attention. Our results show that the detectability of a pair of stimuli decreases with an increase in the distance between stimuli. In addition, the results indicate an increase in attentional effort for different attentional beam widths, and also suggest a decrease in the SD of the internal stimulus representation during the task conditions that require more attention.

Role of remote adaptation in perceived subjective color

Light adaptation to illumination that is presented peripherally changes the subjective color of a central Benham disk stimulus. In our experiments we kept the peripheral illumination achromatic and remote (not even adjacent to the test stimulus). Using a high-frame-rate monitor, we produced the subjective color stimulus, to our knowledge for the first time, on a computer screen in emulation of the Benham disk programs. The resulting changes in the perceived subjective color were as follows: (1) Remote adapting illumination caused a dramatic shift in the perceived subjective color with a span from red to green; (2) there was a trade-off dependence between the area and the intensity of the remote adapting illumination with respect to the perceived color of the test stimulus; (3) the effect of the remote adaptation showed no interocular interaction. This finding suggests that the effect is elicited from a low-level stage in the visual pathway. In addition, we were able to approximate experimentally the spatial profile of the contribution of the remote illumination through the shift in the perceived color. We also found an opposite general trend of color shifts that occurred when either the central stimulus luminance or the remote illumination was increased. A suggested model for the reversed color shifts trend is discussed.

Spatiotemporal adaptation model for retinal ganglion cells

An adaptation model for the level of the ganglion cell in the retina is presented. The model assumes separate adaptation mechanisms for each of the receptive field (RF) regions, i.e., before edge detection. According to the model, the decay in the response time course of each RF region reflects its adaptation process. A mathematical description of adaptation that includes its temporal properties is developed through the change in the semisaturation constant theta in the Naka-Rushton equation. The model and its simulations show a good agreement with a wide variety of physiological studies.

Attentional effort modulated by task difficulty

It has been assumed that stimulus discrimination in a visual task is performed with fixed attentive effort. Here we show that attention to the same pair of stimuli can be modulated by varying the task difficulty when a task requires the discrimination of only a small number of different stimuli. We used a matching-to-sample paradigm, where a test stimulus is presented after a sample stimulus. When both stimuli Gabor gratings have identical orientations ("matching" trial) the required response is different from when they have different orientations ("non-matching" trial). The task difficulty was manipulated by changing the orientation difference between sample and test stimuli for non-matching trials. Difficult non-matching probe trials were embedded within an easy block of trials (easy environment), and vice versa for easy probe trials. Detectability (d') differences for the same pairs of stimuli (probe trials) in the two environments were calculated as a measure for change in attentional effort, regardless of changes in likelihood ratios (beta). Our results show an increase in d' during the difficult task, for both types of probe trials, in paradigms that contained a small number of stimulus combinations. Thus a modulation in attentional effort along a single discrimination dimension is revealed. However it is restricted by the number of stimulus combinations, due to the limited capacity of the attention available for each stimulus combination.

Model of local velocity in the primary visual cortical cells

A motion model for the early stages of motion processing in the visual cortex that focuses on velocity properties is presented. The model presents analytically the correlation between the velocity tuning curve and various cell parameters. The building block for this model is the rebound response, which makes possible the detection of spatial and temporal edges. The model suggests that adjacent subunits in the primary cortical cells display different strengths in their rebound responses, and thus a synergistic response is evoked in the preferred direction. The analysis deals separately with the two cutoff points of the velocity tuning curves. The model predicts a linear relation between the low cutoff point and the receptive-field size and an inverse correlation with the integration time. The high cutoff point is inversely correlated with the cell threshold.

Effect of degree of uniformity on predicted visual cortical response tuning curves

The different cortical visual cells exhibit a large repertoire of responses to sinusoidal gratings, depending on their receptive field structure and the stimulation parameters. It has been shown previously that the tuning curves and histogram shapes of cell responses are affected by subunit distances. One receptive field model (Spitzer and Hochstein 1985b) incorporated subunit distance but assigned it as a constant parameter, for ease of calculation. Here we investigate different tuning curve properties of various primary cortical cell types during testing of 10 deg of nonuniform distances of the receptive fields' subunits. The effect of nonuniformity was compared for average responses, tuning curve shapes, maximum peak responses, and bandwidths across four cell types of different sizes. The shapes and other properties of tuning curves are usually found to be retained also when the degree of uniformity is not very high for most of the cell types. In addition, the effect of uniformity is compared across these different response properties. The maximum peak responses of the tuning curve are found to display a lower coefficient of variation than the bandwidth, for all cell types, for most degrees of uniformity.

Spatio-temporal model for subjective colours based on colour coded ganglion cells

We propose a mathematical model for the generation of the subjective colour phenomenon through Benham's disk stimuli. The model relates to the spatial and temporal properties of three colour coded retinal ganglion cells: L+/M-, M+/L- and S-/(L+M)+ [or (L+M)-/S+]. It is suggested that the phenomenon is based on both the opponent mechanisms in the cells' receptive fields, and the "rebound response"--a common cell response to turning off of an inhibitory stimulus (nonlinear cell dynamics). A physiological mechanism is suggested for this response. The integrated cell responses to Benham disk-stimuli create imbalances between the colour pathways that are interpreted as actual colours. The model also predicts the shift in the perceived colours when the disk rotation rate is varied.

A model for the early stages of motion processing based on spatial and temporal edge detection by X-cells

A model for the early stages of motion processing in the visual cortex is presented. The 'building block' for this model is the 'rebound response', which is the neuronal response evoked when a sufficient inhibitory stimulus is turned off. This response enables detection of temporal changes when the stimulus involves spatial changes. The model suggests that adjacent subunits in primary cortical cells have different weight functions for rebound responses, and thus a synergistic type of response is evoked in the preferred direction, which is predicted for both light and dark stimuli. Predictions of the model for different stimuli and receptive field structures are discussed. It appears to be more economical than previous motion models.

A model for detection of spatial and temporal edges by a single X cell

When an inhibitory visual stimulus is turned off, an increased rate of spike discharge is evoked which we term the "rebound response". This response exists as a part of different cell responses from the retina to the cortex. The rebound response, with its temporal dependence on stimulus parameters, has not been previously considered in models. Here we present such a model, and also show its dependence on stimulus duration and its turning off rate. The rebound response enables detection of temporal changes when a visual stimulus involves spatial changes. The temporal change detection is affected by the actual stimulus duration, which can also be seen as a cell memory operation.

Task difficulty: ignoring, attending to, and discriminating a visual stimulus yield progressively more activity in inferior temporal neurons

To study the influence of task difficulty on the stimulus-elicited responses of inferior temporal (IT) neurons, the stimulus-elicited responses of 64 neurons were recorded from IT cortex of three rhesus monkeys while they performed three behavioral tasks-an irrelevant-stimulus task, a stimulus detection task, and a stimulus discrimination task. The monkey could ignore the stimulus entirely in the irrelevant-stimulus task, was required only to detect stimulus dimming in the stimulus detection task, and was required to attend to specific properties of the stimulus in the discrimination task. The excitatory responses in the discrimination and stimulus detection tasks were larger than those in the irrelevant-stimulus task (61% and 33%, respectively, of the individual differences were significant), and excitatory responses in the discrimination task were larger than those in the detection task (49% of the individual differences reached significance). Twenty percent of the stimulus presentations elicited inhibitory responses that were followed by off-responses. The off-responses were modulated by the tasks in the same order as the excitatory on-responses. Assuming that the off-response strengths indicate the depth of the stimulus-induced inhibition, these results suggest that inhibitory responses were influenced across these tasks in a manner similar to the excitatory responses. When the neuronal responses were related to the difficulties of these tasks, both the response strength and errors were seen to be least during the irrelevant-stimulus task and greatest during the discrimination task. This relationship suggests that the visual responsiveness of IT neurons is related to the degree of attention the animal pays to the stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal encoding of two-dimensional patterns by single units in primate primary visual cortex. I. Stimulus-response relations

1. Previously we developed a new approach for investigating visual system neuronal activity in which single neurons are considered to be communication channels transmitting stimulus-dependent codes in their responses. Application of this approach to the stimulus-response relations of inferior temporal (IT) neurons showed that these carry stimulus-dependent information in the temporal modulation as well as in the strength of their responses. IT cortex is a late station in the visual processing stream. Presumably the neuronal properties arise from the properties of the inputs. However, the discovery that IT neuronal spike trains transmit information in stimulus-dependent temporally modulated codes could not be assumed to be true for those earlier stations, so the techniques used in the earlier study were applied to single-striate cortical neurons in the studies reported here. 2. Single-striate cortical neurons were recorded from three awake, fixating rhesus monkeys. The neurons were stimulated by two sets of patterns. The first set was made up of 128 black-and-white patterns based on a complete, orthogonal set of two-dimensional Walsh-Hadamard functions. These stimuli appear as combinations of black-and-white rectangles and squares, and they fully span the range of all possible black-and-white pictures that can be constructed in an 8 x 8 grid. Except for the stimulus that appeared as an all-white or all-black square, each stimulus had equal areas of white and black. The second stimulus set was made up of single bars constructed in the same 8 x 8 grid as the Walsh stimuli. These were presented both as black against a gray background and white against a gray background. The stimuli were centered on the receptive field, and each member of the stimulus set was presented once before any stimulus appeared again. 3. The responses of 21 striate cortical neurons were recorded and analyzed. Two were identified as simple cells and the other 19 as complex cells according to the criteria originally used by Hubel and Wiesel. The stimulus set elicited a wide variety of response strengths and patterns from each neuron. The responses from both the bars and the Walsh set could be used to differentiate and classify simple and complex cells. 4. The responses of both simple and complex cells showed striking stimulus-related strength and temporal modulation. For all of the complex cells there were instances where the responses to a stimulus and its contrast-reversed mate were substantially different in response strength or pattern, or both.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased attention enhances both behavioral and neuronal performance

Single cells were recorded from cortical area V4 of two rhesus monkeys (Macaca mulatta) trained on a visual discrimination task with two levels of difficulty. Behavioral evidence indicated that the monkeys' discriminative abilities improved when the task was made more difficult. Correspondingly, neuronal responses to stimuli became larger and more selective in the difficult task. A control experiment demonstrated that changes in general arousal could not account for the effects of task difficulty on neuronal responses. It is concluded that increasing the amount of attention directed toward a stimulus can enhance the responsiveness and selectivity of the neurons that process it.

Temporal encoding of two-dimensional patterns by single units in primate inferior temporal cortex. I. Response characteristics

We seek a general approach to determine what stimulus features visual neurons are sensitive to and how those features are represented by the neuron's responses. Because lesions of inferior temporal (IT) cortex interfere with a monkey's ability to perform pattern discrimination tasks we studied IT neurons. Previous single-unit studies have shown that IT neurons sometimes respond more strongly to complex stimuli (brushes, hands, faces) than to simple stimuli (bars, slits, edges). However, it is not known how specific stimulus parameters are represented by responses. We studied the responses of IT neurons in alert behaving monkeys to a large set of two-dimensional black and white patterns. The stimulus set was based on 64 Walsh functions that can be used to represent any picture with a resolution of one part in eight along each of two dimensions. The responses to these stimuli spanned a continuum from inhibition to strong excitation. A statistical test showed that the spike count was determined by which Walsh stimulus was presented. Hence, these stimuli form an adequate set for testing IT neurons. The responses showed temporal modulation of the spike train that could not be represented by a change in the spike count alone. Examples of this modulation were changes in latency, changes in the duration of the response, and alternating periods of excitation and inhibition. This temporal modulation may be important in representing stimulus parameters. The next paper in this series develops a method for quantifying this temporal modulation and shows that it is dependent on the stimulus. The third paper in this series shows that this temporal modulation contains more information about stimulus parameters than is contained in the spike count alone.

Visual receptive fields of cat cortical neurons lack the distinctive geniculate Y cell signature

Retinal ganglion and lateral geniculate cells of the primary visual pathway may be categorized as X or Y according to their spatial linearity summation properties. In particular, Y cell spatial-frequency transfer functions have a distinctive spatial--linearity signature. Their receptive fields comprise a temporally and spatially linear mechanism (center plus antagonistic surround) that responds to relatively low spatial frequency stimuli, and a temporally nonlinear mechanism, coextensive with the linear mechanism, that--though broad in extent--responds best to high spatial-frequency stimuli. This component exhibits spatially nonlinear properties. We looked for this Y cell signature in cat visual cortical cells. If the predominant excitatory input to a certain type of cortical cell were from lateral geniculate nucleus (LGN) Y cells, we would expect to find the Y cell signature underlying the cortical cell transfer function. However, we have not found this Y cell signature in any of the cortical cell studied, and in particular not in the nonlinear complex cells. This would suggest that cortical spatial summation nonlinearities are not functionally derived from input Y cell nonlinearities.

A complex-cell receptive-field model

The time course of the response of a single cortical neuron to counterphase-grating stimulation may vary as a function of stimulation parameters, as shown in the preceding paper (19). The poststimulus-time histograms of the response amplitudes against time are single or double peaked, and where double peaked, the two peaks are of equal or unequal amplitudes. Furthermore, the spatial-phase dependence of cortical complex-cell responses may be a function of spatial frequency, so that the receptive field appears to have linear spatial summation at some spatial frequencies and nonlinear spatial summation at others (19). In the first part of this paper, we analyze a model receptive field that displays this behavior, and in the second part experimental data are presented and analyzed with regard to the model. The model cortical receptive field in its simplest form contains (two rows) of geniculate X-cell-like, DOG (difference-of-Gaussians)-shaped, center-surround antagonistic, circular-input subunits. We propose nonlinear summation between these two subunits, by introducing a half-wave rectification stage before pooling. The model is tested for the responses it predicts for the application of counterphase-grating stimulation. This simple model predicts the appearance of three response forms as a function of counterphase-stimulation parameters. At periodic spatial frequencies the expected-response histogram has a single peak, whose amplitude has a sinusoidal dependence on spatial phase. At spatial frequencies halfway between these, the expected-response histogram has two equal peaks whose amplitudes have a full-wave rectified sinusoidal dependence on spatial phase. At all intermediate spatial frequencies the expected-response histogram has a "mixed" form; the histogram appears sometimes with one peak, sometimes with two equal peaks, and generally with two peaks of unequal amplitude, as a function of spatial phase. Null responses are expected to appear at specific spatial phases only for the periodic spatial frequencies that give "pure" response time courses as in paragraph 5 above, and not in the more common mixed response case of paragraph 6. The analysis procedure described in the preceding paper (19) is used, separating the odd and even Fourier components of the response histograms reflecting the receptive-field intrasubunit linear summation and intersubunit nonlinear summation, respectively. We propose that this model may be used as a working hypothesis for the analysis of these aspects of the various cortical receptive-field types. Experimental data are described and discussed in terms of the model.(ABSTRACT TRUNCATED AT 400 WORDS)

Simple- and complex-cell response dependences on stimulation parameters

We studied the response time course and amplitude dependence on stimulation parameters in cat cortical visual neurons to determine their receptive-field spatial-summation characteristics. Response poststimulus time (PST) histograms of cortical simple cells to contrast-reversal grating stimulation generally have a single peak for each stimulus temporal cycle, though the responses appear rectified. In response to contrast-reversal grating stimulation the general PST histogram time course for complex cells is two peaks, though often these peaks are of different amplitudes. The time course of complex-cell responses, and the ratio of these two response peaks often varies with stimulation parameters. The appearance of a single response peak in simple cells is reflected in the dominance of the odd harmonic Fourier portion, whereas the half-wave rectification leads to a considerable even harmonic portion. Still, this even portion is never significantly greater than the odd portion. When complex cell PST histograms have two nearly equal peaks, Fourier transformation reveals almost only even harmonic components. When the histogram contains two peaks of unequal amplitude Fourier analysis reveals large odd and even components. An even:odd Fourier harmonic portion ratio larger than 1 may be seen as a defining characteristic of complex cells, differentiating them from simple cells. Histograms with two unequal peaks appear "mixed," containing something of the "pure" single-peaked response and something of the pure double-peaked response. The degree to which the response is mixed may be measured by the ratio of the even:odd portion amplitudes. There is a great degree of variability with stimulation parameters (both spatial phase and spatial frequency) of the time course of mixed responses as opposed to the case of responses that have two equal peaks independent of stimulation grating phase and frequency. In both simple and complex cells there is a close coincidence of the spatial frequency ranges over which the even and odd portions are substantial, though many complex cells show a periodic variation of the even:odd portions ratio. This spatial-frequency dependence differs from that of LGN Y-cells where the odd portion dominates at low spatial frequencies and the even portion at high spatial frequencies. The ratio of even-to-odd portion cut-off is close to 3:1 in all Y-cells, a characteristic we did not find in cortical simple or complex cells. We suggest, therefore, that the nonlinearity of these complex cells does not derive from that of Y-cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Zero-crossing detectors in primary visual cortex?

David Marr and others have hypothesized that the visual system processes complex scene information in stages, the first of which involves the detection of light intensity edges or "zero-crossings" (Marr, 1982). Ideal zero-crossing detector mechanisms have been described and modeled in terms of their possible physiological implementation (Marr and Hildreth, 1980; Poggio, 1983). We now present evidence of visual cortical receptive fields which resemble in spatial organizational terms the requirements of zero-crossing analysis. The linear and nonlinear summation within and between the receptive field subunits are described and compared with predicted processes. The relative subunit sizes and separations are analyzed in these terms. Our findings support the notion that receptive fields may correspond with zero-crossing filters rather than zero-crossing detector gates.