Responses Of Neurons In Area Research Articles

Ventromedial Prefrontal Cortex Tracks Multiple Environmental Variables during Search.

To make efficient foraging decisions, we must combine information about the values of available options with nonvalue information. Some accounts of ventromedial PFC (vmPFC) suggest that it has a narrow role limited to evaluating immediately available options. We examined responses of neurons in area 14 (a putative macaque homolog of human vmPFC) as 2 male macaques performed a novel foraging search task. Although many neurons encoded the values of immediately available offers, they also independently encoded several other variables that influence choice, but that are conceptually distinct from offer value. These variables include average reward rate, number of offers viewed per trial, previous offer values, previous outcome sizes, and the locations of the currently attended offer. We conclude that, rather than serving as specialized economic value center, vmPFC plays a broad role in integrating relevant environmental information to drive foraging decisions.SIGNIFICANCE STATEMENT Decision makers must often choose whether to take an immediately available option or continue to search for a better one. We hypothesized that this process, which is integral to foraging theory, leaves neural signatures in the brain region ventromedial PFC. Subjects performed a novel foraging task in which they searched through differently valued options and attempted to balance their reward threshold with various time costs. We found that neurons not only encode the values of immediately available offers, but multiplexed these with environmental variables, including reward rate, number of offers viewed, previous offer values, and spatial information. We conclude that vmPFC plays a rich role in encoding and integrating multiple foraging-related variables during economic decisions.

Sensitivity to relative disparity in early visual cortex of pigmented and albino ferrets

To investigate binocular interactions as the neuronal substrate for disparity sensitivity in the ferret (Mustela putorius furo), we measured the effects of relative horizontal disparities on responses of neurons in areas 17 and 18 of the visual cortex. Stimulation by moving bars and sinusoidal gratings showed that about half of our sample in pigmented ferrets was sensitive to relative horizontal disparity. This also included many neurons, which were classified as only monocularly activated when testing either eye alone. However, the tuning width was about two or three times coarser (median tuning width 4 degrees of visual angle) than that in the cat. In albino ferrets, only 8% of the neurons in the early visual cortex displayed some sort of disparity-dependent binocular interactions, but none could be clearly identified as relative disparity-coding neuron.

Modulatory influence of feedback projections from area 21a on neuronal activities in striate cortex of the cat.

We have examined the influence of 'feedback' projections from extrastriate visual cortical area 21a on the responses of neurons in area 17 of the cat, by cooling area 21a to 5-10 degrees C while the temperature over the recording sites was kept at 36 degrees C. Orientation, direction and length selectivities as well as contrast sensitivity were tested before and during cooling and after rewarming of area 21a. Overall, for the sample of cells recorded from the part of area 17 visuotopically corresponding to area 21a, the 'spontaneous' activity (at background illumination of 1 cd/m(2)) and the responsiveness to visual stimuli of standard contrast (15) were significantly reduced by inactivation of area 21a. In about half of the cells inactivation of area 21a affected substantially the sharpness of orientation-tuning. However, only in a minority of the cells were the direction or length selectivities significantly affected by inactivation of area 21a. Thus (i) the feedback projections from area 21a appear to exert mainly an excitatory influence on the background activity and responsiveness of area 17 cells and (ii) only in subgroups of area 17 cells does the feedback activity originating from area 21a appear to modulate specifically certain receptive field properties.

Visual Responses of Neurons in the Middle Temporal Area of New World Monkeys after Lesions of Striate Cortex

In primates, lesions of striate cortex (V1) result in scotomas in which only rudimentary visual abilities remain. These aspects of vision that survive V1 lesions have been attributed to direct thalamic pathways to extrastriate areas, including the middle temporal area (MT). However, studies in New World monkeys and humans have questioned this interpretation, suggesting that remnants of V1 are responsible for both the activation of MT and residual vision. We studied the visual responses of neurons in area MT in New World marmoset monkeys in the weeks after lesions of V1. The extent of the scotoma in each case was estimated by mapping the receptive fields of cells located near the lesion border and by histological reconstruction. Two response types were observed among the cells located in the part of MT that corresponds, in visuotopic coordinates, to the lesioned part of V1. Many neurons (62%) had receptive fields that were displaced relative to their expected location, so that they represented the visual field immediately surrounding the scotoma. This may be a consequence of a process analogous to the reorganization of the V1 map after retinal lesions. However, another 20% of the cells had receptive fields centered inside the scotoma. Most of these neurons were strongly direction-selective, similar to normal MT cells. These results show that MT cells differ in their responses to lesioning of V1 and that only a subpopulation of MT neurons can be reasonably linked to residual vision and blindsight.

Spatio-temporal frequency separability in area 18 neurons of the cat

Spatial frequency tuning curves were obtained at a series of temporal frequencies and/or velocities from neurons of Areas 17 and 18 of the cat, in order to assess the interaction between these spatial and temporal parameters. In the great majority of instances, for neurons in both Areas 17 and 18, we found the optimal spatial frequency invariant with the temporal frequency or velocity at which a neuron was tested. Thus in this respect, responses of neurons in Area 18, like those of Area 17, are separable in their dependence on spatial and temporal frequency.

The representation of contrast and other stimulus parameters by single neurons in area 17 of the cat.

The responses of neurons in area 17 were tested as a function of various stimulus parameters. The thresholds of individual cortical neurons were at contrasts between 0.01 and 0.1 (increment of 0.5 X 10(-1) cd/m-2 on a background of 3 cd/m-2), the dynamic ranges were 1.0-2.0 log units of increment, and all cells showed a response decrease at increments above a certain maximum (supersaturation response). The averaged contrast/response curve for all neurons was S-shaped in the logarithmic plot, had a dynamic range of 2.5 log units, reached its maximum at a contrast of 0.75 and supersaturated above this level. The contrast/sensitivity curves changed their slope under different stimulus conditions. They became flatter when the non-dominant eye was stimulated as compared to dominant eye stimulation or when the stimulation was done at a non-optimal orientation or direction, and they became steeper when both eyes were stimulated. But the maximum was reached at the same contrast and supersaturation was seen above maximum contrast no matter whether a cell was strongly (e.g. binocular stimulation at optimal orientation) or weakly excited (non-dominant or non-optimal orientation stimulation). After normalization, the averaged population contrast/response curves were virtually identical at all stimulus conditions. It was concluded, that range as well as maximum and supersaturation of cortical contrast/response curves are determined before the input reaches the cortex, and that the cortical cells summate, essentially, linearly. The findings furthermore demonstrate that the supersaturation of the cortical input must be due to subtractive inhibition, and that the same is true for the orientation sensitive inhibition in the cortex itself. Both, the peripheral contrast and the cortical orientation dependent inhibition cannot be explained by multiplicative inhibition. The fact, that the responses of neurons depend on many variables relativates their significance for feature representation.

Neuronal responses in area 7 of the cat cortex during defensive conditioning

Responses of neurons in area 7 of the parietal association cortex during and after formation of a defensive conditioned reflex to sound were recorded in waking cats. Changes in spike responses of the neurons as a result of the onset of conditioned reflex limb movements were observed in 68% of neurons. Spike responses of neurons formed as a result of learning appeared only if conditioned-reflex limb movements appeared, and they were not observed if, for some reason or other, movements were absent after presentation of the positive conditioned stimulus or on extinction of the reflex. Responses of 46% neurons to conditioned stimulation preceded the conditioned-reflex motor responses by 50–450 msec. The remaining responding neurons were recruited into the response after the beginning of movement. Characteristic spike responses of neurons to the conditioned stimulus appeared 500–900 msec before the beginning of movement and, in the case of appearance of special, "prolonged" motor responses of limb withdrawal, evoked by subsequent reinforcing stimulation.

Mechanism of directional sensitivity of receptive fields in the cat visual cortex

Responses of directional-sensitive neurons in area 17 of the cat's cortex were studied to presentation of two flashing bars of light, one located in the center of the field, the other in the inhibitory zone relative to the center. The order of activation of the stimuli and the time interval between them could be varied; presentation of two bars was thus the analog of a moving stimulus. The inhibitory off-zone located at the entrance to the field from the side of the optimal direction of movement was found to have an initial inhibitory phase, followed by a phase of disinhibition, and again by a second inhibitory phase. Presentation of the bars with different time intervals in cases when stimulation of the center of the field coincided in time with one of the inhibitory phases, led to inhibition of the response, but if stimulation coincided with the phase of disinhibition, it led to facilitation. Phases of disinhibition were not found in the inhibitory zone located at the entrance to the field along the course of a nonoptimal direction of movement. The importance of the temporal characteristics of inhibitory zones for the appearance of directional sensitivity of visual courtical neurons is discussed.

Linear and non-linear properties of simple and complex receptive fields in area 17 of the cat visual cortex. A model of the field.

Responses of neurones in area 17 of the cat visual cortex to gratings with sinusoidal distribution of luminance were studied. Impulse responses of the simple fields were found to be modulated by gratings passing the field. The complex fields proved to be of three types: with modulated responses, unmodulated responses, and with modulated responses against unmodulated background. Amplitudephase characteristic measured were inverse Fourier transformed to obtain the field's weighting function. Simultaneously the amplitude-phase characteristic was reconstructed from the weighting function from the responses to edges and bars, with the use of the Fourier transform. Cross-comparison of the reconstructed amplitudephase characteristics and the weighting functions showed that a receptive field has some linear properties but, strictly considered, is a non-linear system. Simple fields display the largest degree of linearity. The weighting function of a simple field does practically not depend on the stimulus velocity. At lowest velocities, however, there additional waves in the weighting function can appear, which results in a narrower bandwidth. The higher the complexity of the field that is to say the greater the number of subfields of the preceding level are, the greater are the deviations from linearity until it is completely lost in some complex fields with the unmodulated type of response. As linear methods are inadequate for dealing with cortical receptive fields, their qualitative identification was performed in model experiments on a computer. A model was fitted to observations by varying the parameters of the model for a receptive field, such as the number and weights of subfields, the extent of their spatial overlap, the time constant of excitatory and inhibitory processes and detection level. The evidence obtained suggest that the receptive fields of the visual cortex form a system of operators which perform the expansion of the image on non-classical pattern. Such an expansion can be termed a piecewise quasi-Fourierdescription. Although receptive fields of the visual cortex have been studied in a great number of investigations, the properties of the fields as described by linear analysis are as yet imperfectly understood. In the present paper we have studied responses of simple and complex receptive fields in area 17 of the cat visual cortex to moving gratings of different spacial frequency with sinusoidal distribution of luminance. The amplitudephase characteristics obtained were inverse Fourier transformed, and the weighting functions reconstructed were compared with the weighting functions obtrined from responses to bars or edges moved through the field. In a similar manner, the weighting function obtained from the responses to bars or edges were Fourier transformed and the corresponding frequency characteristics were compared with those experimentally obtained. Such cross-comparison allowed us to make estimates of linear and non-linear properties of a field to reveal sources of non-linearity. Based on these results we propose a model of the field and discuss a possible role of the field in processing of visual information.

Responses of neurons in area 7 of the parietal cortex to objects of different significance

Responses of neurons in area 7 of the parietal cortex to objects of different significance

Estimation of the length of the traveling wave envelope from response areas of neurons in the cochlear complex of the cat

Plots of discharge rate vs frequency at a number of intensities were determined for 163 neurons. Presumably the bandwidth of these response areas is determined by the shape of the traveling wave envelope on the basilar membrane. According to Greenwood, the distance from the peak to the apical foot of the wave is approximately constant [D. D. Greenwood, J. Acoust. Soc. Am. 34, 1364–1369 (1962)]. Using his frequency-distance function for the cat, we have calculated for each response area the distance along the basilar membrane between the locations for the best frequency and the upper cutoff frequency. When this “critical distance” (c.d.) is plotted versus best frequency for all units, c.d. tends to increase as best frequency declines below about 2 kHz. In order to obtain relatively constant c.d. throughout the entire frequency range, it is necessary to revise Greenwood's function such that the distances for the lower octaves deviate even more from a logarithmic representation than suggested by his original curve. [Supported by NIH grants NS 06225 ,and NS 12732.]

A single neuron activity in the secondary cortical auditory area in the cat.

Discharges of a single neuron in response to tone bursts or continuous pure tones were studied in the secondary cortical auditory area of the cats under very light general anesthesia by means of hyperfine microelectrode technique. Difference of responses of a single neuron to sound stimulation was found between the primary and the secondary areas.(1) Small evoked potentials by tone burst stimulation were recorded from the regions adjacent to the primary area while in the remaining regions they were hardly found.(2) The thresholds of all neurons examined for any stimulating sounds were much higher in the secondary area than in the primary area.(3) The latencies of responses to tone bursts were generally much longer, though variable, in the secondary area than in the primary area.(4) Most neurons in the secondary area responded only to strong continuous pure tones and not to tone bursts alone. Their responsive frequency ranges were very wide. Sometimes, the prolonged after-discharges were observed.(5) Neurons were also encountered responding to tone burst alone. Their responses had the same feature as those of the primary area.(6) Measurements were done on the response areas of neurons. Some of them showed a characteristic properties similar to those in the primary area, while the others were different, the areas being wider and not having the distinct characteristic frequency.(7) By two sound stimulation facilitatory and suppressive interactions were observed on the single neuronal responses just as in the case of the primary area.(8) Tonotopic localisation was not clearly found in this area, because many neurons explored had no distinct characteristic frequencies. Even on the neurons having a narrow response area the situation was very complicated. In contrast to the primary area neurons were easy to be recorded from the lower half of the posterior ectosylvian gyrus.(9) From the results described above it may be concluded that neurons of the secondary area do not play an important role for the analysis of sound, i.e for the discrimination of pitch and intensity of sound, but rather have integrative functions for analyzed component sounds.