Adjacent visual cortical complex cells share about 20% of their stimulus-related information

The responses of adjacent neurons in inferior temporal (IT) cortex carry signals that are to a large degree independent (Gawne and Richmond, 1993). Adjacent primary visual cortical neurons have similar orientation tuning (Hubel and Wiesel, 1962, 1968), suggesting that their responses might be more redundant than those in IT. We recorded the responses of 26 pairs of adjacent complex cells in the primary visual cortex of two awake monkeys while using both a set of 16 bar-like stimuli, and a more complex set of 128 two-dimensional patterns. Linear regression showed that 40% of the signal variance of one neuron was related to that of the other when the responses to the bar-like stimuli were considered. However, when the responses to the two-dimensional stimuli were included in the analysis, only 19% of the signal variance of one neuron was related to that of the adjacent one, almost exactly the same results as found in IT. An information theoretic analysis gave similar results. We hypothesize that this trend toward independence of information processing by adjacent cortical neurons is a general organizational strategy used to maximize the amount of information carried in local groups.

Neural signals in the monkey ventral striatum related to motivation for juice and cocaine rewards

1. The results of neuropsychological, neuropharmacological, and neurophysiological experiments have implicated the ventral striatum in reward-related processes. We designed a task to allow us to separate the effects of sensory, motor, and internal signals so that we could study the correlation between the activity of neurons in the ventral striatum and different motivational states. In this task, a visual stimulus was used to cue the monkeys as to their progress toward earning a reward. The monkeys performed more quickly and with fewer mistakes in the rewarded trials. After analyzing the behavioral results from three monkeys, we recorded from 143 neurons from two of the monkeys while they performed the task with either juice or cocaine reward. 2. In this task the monkey was required to release its grip on a bar when a small visual response cue changed colors from red (the wait signal) to green (the go signal). The duration of the wait signal was varied randomly. The cue became blue whenever the monkey successfully responded to the go signal within 1 s of its appearance. A reward was delivered after the monkey successfully completed one, two, or three trials. The schedules were randomly interleaved. A second visual stimulus that progressively brightened or dimmed signaled to the monkeys their progress toward earning a reward. This discriminative cue allowed the monkeys to judge the proportion of work remaining in the current ratio schedule of reinforcement. Data were collected from three monkeys while they performed this task. 3. The average reaction times became faster and error rates declined as the monkeys progressed toward completing the current schedule of reinforcement and thereby earning a reward, whereas the modal reaction time did not change. As the duration of the wait period before the go signal increased, the monkeys reacted more quickly but their error rates scarcely changed. From these results we infer that the effects of motivation and motor readiness in this task are generated by separate mechanisms rather than by a single mechanism subserving generalized arousal. 4. The activity of 138 ventral striatal neurons was sampled in two monkeys while they performed the task to earn juice reward. We saw tonic changes in activity throughout the trials, and we saw phasic activity following the reward. The activity of these neurons was markedly different during juice-rewarded trials than during correctly performed trials when no reward was forthcoming (or expected). The responses also were weakly, but significantly, related to the proximity of the reward in the schedules requiring more than one trial. 5. The monkeys worked to obtain intravenous cocaine while we recorded 62 neurons. For 57 of the neurons, we recorded activity while the monkeys worked in blocks of trials during which they self-administered cocaine after blocks during which they worked for juice. Although fewer neurons responded to cocaine than to juice reward (19 vs. 33%), this difference was not significant. The neuronal response properties to cocaine and juice rewards were independent; that is, the responses when one was the reward one failed to predict the response when the other was the reward. In addition, the neuronal activity lost most of its selectivity for rewarded trials, i.e, the activity did not distinguish nearly as well between cocaine and sham rewards as between juice and sham rewards. 6. Our results show that mechanisms by which cocaine acts do not appear to be the same as the ones activated when the monkeys were presented with an oral juice reward. This finding raises the intriguing possibility that the effects of cocaine could be reduced selectively without blocking the effects of many natural rewards.

Information flow and temporal coding in primate pattern vision

We perform time-resolved calculations of the information transmitted about visual patterns by neurons in primary visual and inferior temporal cortices. All measurable information is carried in an effective time-varying firing rate, obtained by averaging the neuronal response with a resolution no finer than about 25 ms in primary visual cortex and around twice that in inferior temporal cortex. We found no better way for a neuron receiving these messages to decode them than simply to count spikes for this long. Most of the information tends to be concentrated in one or, more often, two brief packets, one at the very beginning of the response and the other typically 100 ms later. The first packet is the most informative part of the message, but the second one generally contains new information. A small but significant part of the total information in the message accumulates gradually over the entire course of the response. These findings impose strong constraints on the codes used by these neurons.

Decoding cortical neuronal signals: network models, information estimation and spatial tuning

We have studied the encoding of spatial pattern information by complex cells in the primary visual cortex of awake monkeys. Three models for the conditional probabilities of different stimuli, given the neuronal response, were fit and compared using cross-validation. For our data, a feed-forward neural network proved to be the best of these models. The information carried by a cell about a stimulus set can be calculated from the estimated conditional probabilities. We performed a spatial spectroscopy of the encoding, examining how the transmitted information varies with both the average coarseness of the stimulus set and the coarseness differences within it. We find that each neuron encodes information about many features at multiple scales. Our data do not appear to allow a characterization of these variations in terms of the detection of simple single features such as oriented bars.

Cortical feedback increases visual information transmitted by monkey parvocellular lateral geniculate nucleus neurons

We studied the effect of cooling the striate cortex on parvocellular lateral geniculate nucleus (PLGN) neurons in awake monkeys. Cooling the striate cortex produced both facilitation and inhibition of the responses of all neurons, depending on the stimulus presented. Cooling the striate cortex also altered the temporal distribution of spikes in the responses of PLGN neurons. Shannon's information measure revealed that cooling the striate cortex reduced the average stimulus-related information transmitted by all PLGN neurons. The reduction in transmitted information was associated with both facilitation and inhibition of the response. Cooling the striate cortex reduced the amount of information transmitted about all of the stimulus parameters tested: pattern, luminance, spatial contrast, and sequential contrast. The effect of cooling was nearly the same for codes based on the number of spikes in the response as for codes based on their temporal distribution. The reduction in transmitted information occurred because the differences among the responses to different stimuli (signal separation) were reduced, not because the variability of the responses to individual stimuli (noise) was increased. We conclude that one function of corticogeniculate feedback is to improve the ability of PLGN neurons to discriminate among stimuli by enhancing the differences among their responses.

Fast spin-echo imaging of the knee: factors influencing contrast

Conventional T2-weighted spin-echo magnetic resonance imaging of the knee requires a long TR. Fast spin-echo (FSE) imaging can improve acquisition efficiency severalfold by collecting multiple lines of k space for each TR. Compromises in resolution, section coverage, and contrast inevitably result. The authors examined the compromises encountered in FSE imaging of the knee and discuss the variations in image contrast and resolution due to choices of sequence parameters. For short TR/TE knee imaging, FSE does not appear to offer any advantages, since the increased collection efficiency for one section reduces the available number of sections, so that the total imaging time for a given number of sections remains constant relative to conventional spin-echo imaging. For T2-weighted images, considerable time can be saved and comparable quality images can be obtained. This saved time can be usefully spent on increasing both the resolution of the image and its signal-to-noise ratio, while still reducing total acquisition time by a factor of two. The preferred FSE T2-weighted images were acquired with a TR of 4,500 msec, TE of 120 msec, and eight echoes. The available number of sections is compromised, and the sequence remains sensitive to flow artifacts; however, the FSE sequence appears to be promising for knee imaging.

How independent are the messages carried by adjacent inferior temporal cortical neurons?

There are at least three possibilities for encoding information in a small area of cortex. First, neurons could have identical characteristics, thus conveying redundant information; second, neurons could give different responses to the same stimuli, thus conveying independent information; or third, neurons could cooperate with each other to encode more information jointly than they do separately, that is, synergistically. We recorded from 28 pairs of neurons in inferior temporal cortex of behaving rhesus monkeys. Each pair was recorded from a single microelectrode. Both the magnitude and the temporal modulation of the responses were quantified. We separated the responses into signal (average response to each stimulus) and noise (deviation of each response from the average). Linear regression showed that an average of only 18.7% of the magnitude of the signal carried by one neuron could be predicted from the magnitude of the other, and only 22.0% could be predicted by including the temporal modulation. For the noise, the figures were 5.5% and 6.3%, respectively, even less than for the signal. Information theoretic analysis shows that the pairs of neurons we studied carried an average of 20% redundant information. However, even this relatively small amount of redundancy places a severe upper limit on the information that can be transmitted by a neuronal pool. A pool of neurons for which each pair is mutually redundant to extent y can only carry a maximum of 1/y, here five times, as much information as one neuron alone. Information theoretic analysis gave no evidence for the presence of information as a function of both neurons considered together, that is, synergistic codes. Cross-correlation showed that at least 61% of the neuronal pairs shared connections in some manner. Given these shared connections, if adjacent neurons had had identical characteristics, then the noise on the outputs of these neurons would have been highly correlated, and it would not be possible to separate the signal and noise. The severe impact of correlated noise and information redundancy leads us to propose that the processing carried out by these neurons evolved both to provide a rich description of many stimulus properties and simultaneously to minimize the redundancy in a local group of neurons. These two principles appear to be a major constraint on the organization of inferior temporal, and possibly all, cortex.

Prospective study of osseous, articular, and meniscal lesions in recent anterior cruciate ligament tears by magnetic resonance imaging and arthroscopy

Fifty-four patients with anterior cruciate ligament tears that were arthroscopically reconstructed within 3 months of initial injury were prospectively evaluated. Patients with grade 3 medial collateral ligament, lateral collateral ligament, or posterior cruciate ligament tears were excluded. Eighty percent of our patients had a bone bruise present on the magnetic resonance image, with 68% in the lateral femoral condyle. Two of the latter findings--an abnormal articular cartilage signal (P = 0.02) and a thin and impacted subchondral bone (P = 0.03)--had a significant relationship with injury to the overlying articular cartilage. Meniscal tears were found in 56% of the lateral menisci and 37% of the medial menisci. A significant association was present between bone bruising on the lateral femoral condyle and the lateral tibial plateau (P = 0.02). Results of our study support the concept that the common mechanism of injury to the anterior cruciate ligament involves severe anterior subluxation with impaction of the posterior tibia on the anterior femur. Determination of the significance of bone bruising, articular cartilage injury, or meniscal tears will require a long-term followup that includes evaluation for arthritis, stability, and function. These 54 patients represent the first cohort evaluated in this ongoing prospective clinical study.

Role of inferior temporal neurons in visual memory. I. Temporal encoding of information about visual images, recalled images, and behavioral context

1. Lesions of the inferior temporal (IT) cortex selectively hamper monkeys in tasks requiring visual memory. A system that recognizes images must be able to encode a current stimulus, recall the code of a previous stimulus, compare the codes of the two stimuli, and make a decision on the basis of the outcome of the comparison. Therefore, IT neurons must be involved in at least one of these processes. To determine the specific role of IT neurons in visual memory, we measured the information conveyed in the neuronal responses about current patterns, recalled patterns, and behavioral context. 2. Two monkeys were trained to perform a sequential matching task using a set of 32 black and white Walsh patterns. In the course of an experiment, each pattern was presented repeatedly in sample, match, and nonmatch behavioral contexts. While the monkeys were performing the task, we recorded the activity of 76 neurons from area TE of IT. The neuronal responses to the stimuli were converted to spike density functions, and the resultant waveforms were quantified using their principal components. The relationships between the responses and the stimuli were studied using analysis of variance and information theory. 3. The analysis of variance was applied to the neuronal response waveforms using the context (sample, match, or nonmatch) and the patterns of the stimuli as independent variables and the spike count or the coefficients of the principal components as the dependent variables. We found that the waveforms of most neurons were significantly modulated by both the pattern and the context of the stimulus presentation. 4. We also analyzed the stimulus-response relationships using information theory. The input codes were based on the pattern and context of the stimuli, and the output codes were based on the spike count or the principal components of the responses. The neuronal response waveforms were found to convey significant amounts of information about both the pattern and context of the stimuli. Transmitted information was greatest when the response of a neuron was interpreted as a message about the combination of pattern and context. Nevertheless, there was information about context independent of pattern and vice versa. 5. We also used information theory to determine whether the neuronal responses to the second, or test, stimulus conveyed information about the pattern of the first, or sample, stimulus. The input codes were based on the patterns of the sample stimuli, and the output codes were based on the responses to the nonmatch test stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Medial malleolar stress fractures in seven patients: review of the clinical and imaging features

Stress fractures of the medial malleolus were discovered in seven patients, five male and two female subjects aged 16-34 years. All except one were involved in running and jumping athletic activities. Gradual onset of pain over the medial malleolus occurred with repetitive activity. Focal intense increased uptake in the medial malleolus was present on bone scans. Conventional radiography and computed tomography demonstrated the presence of subtle fissures at the junction of the medial malleolus and the tibial plafond, and well-circumscribed lytic lesions were also seen in three patients. Two patients developed a complete fracture of the medial malleolus. Stress fractures of the medial malleolus should be suspected in patients involved in athletic and/or unusual activities who have experienced persistent and unexplained pain over the medial malleolus. Bone scans and radiographs should be obtained for diagnostic purposes in these patients.

Role of inferior temporal neurons in visual memory. II. Multiplying temporal waveforms related to vision and memory

1. In the companion paper we reported on the activity of neurons in the inferior temporal (IT) cortex during a sequential pattern matching task. In this task a sample stimulus was followed by a test stimulus that was either a match or a nonmatch. Many of the neurons encoded information about the patterns of both current and previous stimuli in the temporal modulation of their responses. 2. A simple information processing model of visual memory can be formed with just four steps: 1) encode the current stimulus; 2) recall the code of a remembered stimulus; 3) compare the two codes; 4) and decide whether they are similar or different. The analysis presented in the first paper suggested that some IT neurons were performing the comparison step of visual memory. 3. We propose that IT neurons participate in the comparison of temporal waveforms related to vision and memory by multiplying them together. This product could form the basis of a crosscorrelation-based comparison. 4. We tested our hypothesis by fitting a simple multiplicative model to data from IT neurons. The model generated waveforms in separate memory and visual channels. The waveforms arising from the two channels were then multiplied on a point by point basis to yield the output waveform. The model was fitted to the actual neuronal data by a gradient descent method to find the best fit waveforms that also had the lowest total energy. 5. The multiplicative model fit the neuronal responses quite well. The multiplicative model made consistently better predictions of the actual response waveforms than did an additive model. Furthermore, the fit was better when the actual relationship between the responses and the sample and test stimuli were preserved than when that relationship was randomized. 6. We infer from the superior fit of the multiplicative model that IT neurons are multiplying temporally modulated waveforms arising from separate visual and memory systems in the comparison step of visual memory.

Transient osteoporosis of the hip: clinical and imaging features

Transient osteoporosis of the hip is a form of reflex sympathetic dystrophy characterized by pain, limping, limitation of hip joint motion, and delayed radiographic patchy osteoporosis of the proximal femur. Spontaneous resolution is usually paralleled by radiographic recovery, usually within a few months. We present clinical and imaging features in seven cases of unilateral transient osteoporosis of the hip. In the appropriate clinical setting, conventional radiography will support the diagnosis. The role of more sensitive imaging techniques such as bone scintigraphy and magnetic resonance imaging in the early diagnosis of this disease has yet to be defined.

Diagnostic efficacy of digitized images vs plain films: a study of the joints of the fingers

Four hundred fifteen finger joints from 30 patients were evaluated for the presence of joint-space erosion, narrowing, and degenerative spurring on plain films, low-resolution digitized images (1024 x 840 bytes x 12 bit matrix), and high-resolution digitized images (2048 x 1680 bytes x 12 bit matrix). Three hundred four joints were abnormal. Low- and high-resolution digital images were displayed on a 1K x 1K monitor with the ability to change level, window, orientation, and brightness. Five radiologists interpreted images. The presence or absence of each abnormality was determined by consensus of two skeletal radiologists who did not otherwise participate in the study. Receiver-operating-characteristic analysis was used to obtain an area and a true-positive rate at a 0.10 false-positive rate for each interpreter. Randomized block analysis of variance with interpreters as blocks was used to compare areas and true-positive rates among imaging techniques for each type of abnormality; no statistically significant differences were found. In conclusion, the efficacy of display of digitized images on high- and low-resolution modes is not significantly different from that of plain films in the detection of erosions, joint-space narrowing, or degenerative spurring in small joints of the hands.

Application of an artificial neural network in radiographic diagnosis

The description of 44 cases of bone tumors was used by an artificial neural network to rank the likelihood of 55 possible pathologic diagnoses. The performance of the artificial neural network was compared with the performance of experienced (3 or more years of radiology training) residents and inexperienced (less than 1 year of radiology training) residents. The artificial neural network was trained using descriptions of 110 radiographs of bone tumors with known diagnoses. The descriptions of a separate set of 44 cases were used to test the neural network. The neural network ranked 55 possible pathologic diagnoses on a scale from 1 to 55. Experienced and inexperienced residents also ranked the possible diagnoses in the same 44 cases. Inexperienced residents had a significantly lower mean proportion of diagnoses ranked first or second than did the neural network. Experienced residents had a significantly higher proportion of correct diagnoses ranked first than did the network. Otherwise, a significant difference between the performance of the network and experienced or inexperienced residents was not identified. These results demonstrate that artificial neural networks can be trained to classify bone tumors. Whether neural network performance in classification of bone tumors can be made accurate enough to assist radiologists in clinical practice remains an open question. These preliminary results indicate that further investigation of this technology for interpretation assistance is warranted.

Lateral geniculate neurons in behaving primates. II. Encoding of visual information in the temporal shape of the response

1. We used the Karhunen-Loève (K-L) transform to quantify the temporal distribution of spikes in the responses of lateral geniculate (LGN) neurons. The basis functions of the K-L transform are a set of waveforms called principal components, which are extracted from the data set. The coefficients of the principal components are uncorrelated with each other and can be used to quantify individual responses. The shapes of each of the first three principal components were very similar across neurons. 2. The coefficient of the first principal component was highly correlated with the spike count, but the other coefficients were not. Thus the coefficient of the first principal component reflects the strength of the response, whereas the coefficients of the other principal components reflect aspects of the temporal distribution of spikes in the response that are uncorrelated with the strength of the response. Statistical analysis revealed that the coefficients of up to 10 principal components were driven by the stimuli. Therefore stimuli govern the temporal distribution as well as the number of spikes in the response. 3. Through the application of information theory, we were able to compare the amount of stimulus-related information carried by LGN neurons when two codes were assumed: first, a univariate code based on response strength alone; and second, a multivariate temporal code based on the coefficients of the first three principal components. We found that LGN neurons were able to transmit an average of 1.5 times as much information using the three-component temporal code as they could using the strength code. 4. The stimulus set we used allowed us to calculate the amount of information each neuron could transmit about stimulus luminance, pattern, and contrast. All neurons transmitted the greatest amount of information about stimulus luminance, but they also transmitted significant amounts of information about stimulus pattern. This pattern information was not a reflection of the luminance or contrast of the pixel centered on the receptive field. 5. In addition to measuring the average amount of information each neuron transmitted about all stimuli, we also measured the amount of information each neuron transmitted about the individual stimuli with both the univariate spike count code and the multivariate temporal code. We then compared the amount of information transmitted per stimulus with the magnitudes of the responses to the individual stimuli. We found that the magnitudes of both the univariate and the multivariate responses to individual stimuli were poorly correlated with the information transmitted about the individual stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Lateral geniculate neurons in behaving primates. III. Response predictions of a channel model with multiple spatial-to-temporal filters

1. For the experiments reported in these papers, we recorded the responses of lateral geniculate (LGN) neurons to a large set of two-dimensional, black and white patterns based on Walsh functions and to a set of test stimuli. In the first two papers we reported that these neurons encode stimulus-related information in both the strength and the shape of the response waveforms and that there are more than two independent components in the response. These results cannot be explained by existing models. This paper provides a model of LGN neurons that not only accounts for the foregoing observations, but also yields predictions confirmed by direct tests. 2. The model represents a neuron as a set of three parallel channels. The input to each channel is an array of pixel luminances. Each channel consists of an input nonlinearity cascaded into a linear spatial-to-temporal filter. The output of each channel is a basic waveform, a principal component. The response of the neuron is the sum of the outputs of the three channels. 3. The model accounted for much of the variance in the coefficients of the first three principal components of the neuronal responses to the set of Walsh stimuli. Using parameters derived from the responses of neurons to the Walsh stimuli only, the model also predicted the responses to "center-surround" annuli of different contrasts and mean luminances, as well as to superpositions of pairs of Walsh patterns. The model made statistically significant predictions of the coefficients of two of the principal components of these responses. 4. After the parameters of the model had been fit to reproduce the responses of neurons to the Walsh stimuli, we found that the input nonlinearity of the model was compressed at both the high and low luminance levels. This compression produced response saturation that closely resembled the response saturation of neurons reported in the first paper in this series. Although not absolutely smooth, the spatial filter for the first channel had a dominant excitatory or inhibitory center and an antagonistic surround. Thus this spatial filter accounted for both the center and the surround structures of previous models of LGN receptive fields. There was greater variety in the structures of the spatial filters for the second and third channels, but none had a center-surround organization. Many of the spatial filters for these higher channels contained oriented ridges or valleys. Other spatial filters were dominated by a bipolar pair of pixels. 5. The model of LGN neurons that we present in this paper represents an extension over previous models in four ways. First, the model is capable of explaining the responses of neurons to a wider range of luminances than previous models. Second, the model is capable of explaining the shapes of the response waveforms as well as their magnitudes. Third, the concept of a single receptive field is extended to a series of spatial-to-temporal filters. Fourth, the model suggests that LGN neurons provide a description of both the brightness and the form of a stimulus in their response waveforms.

Lateral geniculate neurons in behaving primates. I. Responses to two-dimensional stimuli

1. Using behaving monkeys, we studied the visual responses of single neurons in the parvocellular layers of the lateral geniculate nucleus (LGN) to a set of two-dimensional black and white patterns. We found that monkeys could be trained to make sufficiently reliable and stable fixations to enable us to plot and characterize the receptive fields of individual neurons. A qualitative examination of rasters and a statistical analysis of the data revealed that the responses of neurons were related to the stimuli. 2. The data from 5 of the 13 "X-like" neurons in our sample indicated the presence of antagonistic center and surround mechanisms and linear summation of luminance within center and surround mechanisms. We attribute the lack of evidence for surround antagonism in the eight neurons that failed to exhibit center-surround antagonism either to a mismatch between the size of the pixels in the stimuli and the size of the receptive field or to the lack of a surround mechanism (i.e., the type II neurons of Wiesel and Hubel). 3. The data from five other neurons confirm and extend previous reports indicating that the surround regions of X-like neurons can have nonlinearities. The responses of these neurons were not modulated when a contrast-reversing, bipartite stimulus was centered on the receptive field, which suggests a linear summation within the center and surround mechanisms. However, it was frequently the case for these neurons that stimuli of identical pattern but opposite contrast elicited responses of similar polarity, which indicates nonlinear behavior. 4. We found a wide variety of temporal patterns in the responses of individual LGN neurons, which included differences in the magnitude, width, and number of peaks of the initial on-transient and in the magnitude of the later sustained component. These different temporal patterns were repeatable and clearly different for different visual patterns. These results suggest that visual information may be carried in the shape as well as in the amplitude of the response waveform.

Concurrent processing and complexity of temporally encoded neuronal messages in visual perception

The intrinsic neuronal code that carries visual information and the perceptual mechanism for decoding that information are not known. However, multivariate statistics and information theory show that neurons in four visual areas simultaneously carry multiple, stimulus-related messages by utilizing multiplexed temporal codes. The complexity of these temporal messages increases progressively across the visual system, yet the temporal codes overlap in time. Thus, visual perception may depend on the concurrent processing of multiplexed temporal messages from all visual areas.

Interactive effects among several stimulus parameters on the responses of striate cortical complex cells

1. Although neurons within the visual system are often described in terms of their responses to particular patterns such as bars and edges, they are actually sensitive to many different stimulus features, such as the luminances making up the patterns and the duration of presentation. Many different combinations of stimulus parameters can result in the same neuronal response, raising the problem of how the nervous system can extract information about visual stimuli from such inherently ambiguous responses. It has been shown that complex cells transmit significant amounts of information in the temporal modulation of their responses, raising the possibility that different stimulus parameters are encoded in different aspects of the response. To find out how much information is actually available about individual stimulus parameters, we examined the interactions among three stimulus parameters in the temporally modulated responses of striate cortical complex cells. 2. Sixteen black and white patterns were presented to two awake monkeys at each of four luminance-combinations and five durations, giving a total of 320 unique stimuli. Complex cells were recorded in layers 2 and 3 of striate cortex, with the stimuli centered on the receptive fields as determined by mapping with black and white bars. 3. An analysis of variance (ANOVA) was applied to these data with the three stimulus parameters of pattern, the luminance-combinations, and duration as the independent variables. The ANOVA was repeated with the magnitude and three different aspects of the temporal modulation of the response as the dependent variables. For the 19 neurons studied, many of the interactions between the different stimulus parameters were statistically significant. For some response measures the interactions accounted for more than one-half of the total response variance. 4. We also analyzed the stimulus-response relationships with the use of information theoretical techniques. We defined input codes on the basis of each stimulus parameter alone, as well as their combinations, and output codes on the basis of response strength, and on three measures of temporal modulation, also taken individually and together. Transmitted information was greatest when the response of a neuron was interpreted as a temporally modulated message about combinations of all three stimulus parameters. The interaction terms of the ANOVA suggest that the response of a complex cell can only be interpreted as a message about combinations of all three stimulus parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Temporal encoding of two-dimensional patterns by single units in primate primary visual cortex. II. Information transmission

1. Previously, we studied how picture information was processed by neurons in inferior temporal cortex. We found that responses varying in both response strength and temporal waveform carried information about briefly flashed stationary black-and-white patterns. Now, we have applied that same paradigm to the study of striate cortical neurons. 2. In this approach the responses to a set of basic black and white pictures were quantified through use of a set of basic waveforms, the principal components (extracted from all the responses of each neuron). We found that the first principal component, which corresponds to the response strength, and others, which correspond to different basic temporal activity patterns, were significantly related to the stimuli, i.e., the stimulus drove both the response strength and its temporal pattern. 3. Our previous study had shown that, when information theory was used to quantify the stimulus-response relation, inferior temporal neurons convey over twice as much information in a response code that includes temporal modulation as in a response code that includes only the response strength. This study shows that striate cortical neurons also carry twice as much information in a temporal code as in a response strength code. Thus single visual neurons at both ends of a cortical processing chain for visual pattern use a multidimensional temporal code to carry stimulus-related information. 4. These results support our multiplex-filter hypothesis, which states that single visual system neurons can be regarded as several simultaneously active parallel channels, each of which conveys independent information about the stimulus.