Energy model for contrast detection: spatiotemporal characteristics of threshold vision

A model for contrast detection of spatiotemporal stimuli is proposed which consists of a spatiotemporal linear filter, an energy device and a threshold device. Assuming the existence of independent intrinsic noise, the probability of stimulus detection was approximated by a Weibull function of the response energy. With this assumption, the stimulus energy is a constant at fixed detection probability. This energy model for contrast detection satisfactorily accounted for the elliptical threshold contours of line pairs at stimulus separations within the range 2–30 min and at stimulus onset asynchronies within the range 20–140 ms. The threshold contour at a large stimulus onset asynchrony (300 ms) was in the form of a rounded square. This finding was explained by assuming that the probability of seeing the line pair was determined by the joint probability that at least one stimulus had been detected. With the energy model, the temporal and spatial autocorrelation functions of the response to a flashed line were evaluated. The autocorrelation functions thus determined were used to predict the temporal contrast sensitivity function to a flickering line stimulus and the spatial contrast sensitivity function to flashed gratings, which were in agreement with the experimental data. The data obtained were fitted adequately by an impulse response approximated by a spatiotemporal Gabor-like function. Received: 08 December 1997 / Accepted in revised form: 26 January 1999     

Neural correlates of novelty detection in pulse-type weakly electric fish

Evoked potentials (EPs) and single unit recordings from various electrosensory-processing regions of several pulse-type gymnotiform species were made to investigate neural activity patterns that could be associated with novelty detection. Whereas the electrosensory afferents and cells in the ELL exhibited only minor changes in response size as stimuli were presented less frequently (novel stimuli), most units studied in the torus semicircularis (TS) showed very strong, increased responsiveness to stimuli presented less frequently relative to stimuli presented persistently (at every EOD event. The responses of the TS were graded with respect to stimulus frequency. The discrimination between novel and persistent stimuli by the TS occurred with stimuli presented transversely or longitudinally with respect to the fish's long axis, and regardless of the timing of the stimulus with respect to the fish's pacemaker-related signal (PS). When electrosensory novelties were presented persistently the responses of the TS rapidly habituated. This may indicate that activity in this region of the TS is novelty related. This novelty-related activity in the TS can be correlated with certain aspects of the fish's behavior, i.e., EOD interval length during a behavioral novelty response. However, TS activity may continue to indicate the occurrence of electrosensory novelties after the behavior has habituated. It is suggested that the novelty-related activity of the TS of these fish is necessary, but not sufficient, for the production of electrosensory novelty-induced behavioral responses. Lesions of the region of the TS containing the rapidly-habituating neurons abolished the electrosensory novelty response, but not that resulting from visual and auditory stimulation.     

Neural correlates of hate

In this work, we address an important but unexplored topic, namely the neural correlates of hate. In a block-design fMRI study, we scanned 17 normal human subjects while they viewed the face of a person they hated and also faces of acquaintances for whom they had neutral feelings. A hate score was obtained for the object of hate for each subject and this was used as a covariate in a between-subject random effects analysis. Viewing a hated face resulted in increased activity in the medial frontal gyrus, right putamen, bilaterally in premotor cortex, in the frontal pole and bilaterally in the medial insula. We also found three areas where activation correlated linearly with the declared level of hatred, the right insula, right premotor cortex and the right fronto-medial gyrus. One area of deactivation was found in the right superior frontal gyrus. The study thus shows that there is a unique pattern of activity in the brain in the context of hate. Though distinct from the pattern of activity that correlates with romantic love, this pattern nevertheless shares two areas with the latter, namely the putamen and the insula.     

Neural correlates of decisions

Once considered the province of philosophy and the behavioral sciences, the process of making decisions has received increasing scrutiny from neurobiologists. Recent research suggests that sensory judgements unfold through the gradual accumulation of neuronal signals in sensory-motor pathways, favoring one alternative over others. Stored representations of the outcome of prior actions activate neurons in many of these same areas during decision-making. The challenge for neurobiologists lies in deciphering how signals from these disparate areas are integrated to form a single behavioral choice and the mechanisms responsible for selecting the appropriate information upon which decisions should be informed in particular contexts.     

Neural correlates of frog calling

Summary A tissue bath incorporating a screen for support of the specimen and an air-lift pump to circulate saline across the screen was designed to provide maximum exposure of isolated frog brainstems ofRana pipiens pipiens to oxygenated saline (Fig. 1). Normal neural correlates of electrically-evoked mating calling were recorded from the region of the pretrigeminal nucleus and the laryngeal nerve in the isolated brainstem (Fig. 3A) and isolated hemi-brainstem (Fig. 2) of the Northern leopard frog. Conspicuous slow-wave activity in the region of the pretrigeminal nucleus supports the possibility that this may be an important integrative area for calling. It appears that the pretrigeminal region is not able, independently, to generate the pulses of the vocal phase of calling. Synchronizing and reinforcing inter-connections between the calling mechanisms of the two sides were identified. The data are summarized in a revised model of mating calling (Fig. 7).This work was supported by NINDS grant NS-06673. The electronic equipment was set up and maintained by Mr. Wayne R. Hudson. I am grateful to Dr. William Van Meter for suggesting the Sylgard for the pinning block and to Dr. Patricia Gallagher for suggesting the saline solution of Phillis and Tebcis.     

Area summation in human vision at and above detection threshold

The initial image-processing stages of visual cortex are well suited to a local (patchwise) analysis of the viewed scene. But the world's structures extend over space as textures and surfaces, suggesting the need for spatial integration. Most models of contrast vision fall shy of this process because (i) the weak area summation at detection threshold is attributed to probability summation (PS) and (ii) there is little or no advantage of area well above threshold. Both of these views are challenged here. First, it is shown that results at threshold are consistent with linear summation of contrast following retinal inhomogeneity, spatial filtering, nonlinear contrast transduction and multiple sources of additive Gaussian noise. We suggest that the suprathreshold loss of the area advantage in previous studies is due to a concomitant increase in suppression from the pedestal. To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size. Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation. Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.     

Neural correlates of behavioral gap detection in the inferior colliculus of the young CBA mouse

The gap detection paradigm is frequently used in psychoacoustics to characterize the temporal acuity of the auditory system. Neural responses to silent gaps embedded in white-noise carriers, were obtained from mouse inferior colliculus (IC) neurons and the results compared to behavioral estimates of gap detection. Neural correlates of gap detection were obtained from 78 single neurons located in the central nucleus of the IC. Minimal gap thresholds (MGTs) were computed from single-unit gap functions and were found to be comparable, 1–2 ms, to the behavioral gap threshold (2 ms). There was no difference in MGTs for units in which both carrier intensities were collected. Single unit responses were classified based on temporal discharge patterns to steady-state noise bursts. Onset and primary-like units had the shortest mean MGTs (2.0 ms), followed by sustained units (4.0 ms) and phasic-off units (4.2 ms). The longest MGTs were obtained for inhibitory neurons (xˉ = 14 ms). Finally, the time-course of behavioral and neurophysiological gap functions were found to be in good agreement. The results of the present study indicate the neural code necessary for behavioral gap detection is present in the temporal discharge patterns of the majority of IC neurons. Accepted: 6 February 1997     

Neural correlates of categories and concepts

The ability to readily adapt to novel situations requires something beyond storing specific stimulus-response associations. Instead, many animals can detect basic characteristics of events and store them as generalized classes. Because these representations are abstracted beyond specific details of sensory inputs and motor outputs, they can be easily generalized and adapted to new circumstances. Explorations of neural mechanisms of sensory processing and motor output have progressed to the point where studies can begin to address the neural basis of abstract, categorical representations. Recent studies have revealed their neural correlates in various cortical areas of the non-human primate brain.     

Neural correlates of visual motion prediction

Predicting the trajectories of moving objects in our surroundings is important for many life scenarios, such as driving, walking, reaching, hunting and combat. We determined human subjects' performance and task-related brain activity in a motion trajectory prediction task. The task required spatial and motion working memory as well as the ability to extrapolate motion information in time to predict future object locations. We showed that the neural circuits associated with motion prediction included frontal, parietal and insular cortex, as well as the thalamus and the visual cortex. Interestingly, deactivation of many of these regions seemed to be more closely related to task performance. The differential activity during motion prediction vs. direct observation was also correlated with task performance. The neural networks involved in our visual motion prediction task are significantly different from those that underlie visual motion memory and imagery. Our results set the stage for the examination of the effects of deficiencies in these networks, such as those caused by aging and mental disorders, on visual motion prediction and its consequences on mobility related daily activities.     

Neural correlates of the attentional blink

Attending to a visual event can lead to functional blindness for other events in the visual field. This limit in our attentional capacities is exemplified by the attentional blink (AB), which refers to the transient but severe impairment in perceiving the second of two temporally neighboring targets. Using functional magnetic resonance imaging (fMRI), we observed predominantly right intraparietal and frontal cortex activations associated with the AB. We further demonstrate that an AB can be elicited by both temporal and spatial distractor interference on an attended target and that both of these interference mechanisms activate the same neural circuit. These results suggest that a (right) parietofrontal network previously implicated in attentional control and enhancement is also a locus of capacity-limited processing of visual information.     

Neural correlates of economic game playing

The theory of games provides a mathematical formalization of strategic choices, which have been studied in both economics and neuroscience, and more recently has become the focus of neuroeconomics experiments with human and non-human actors. This paper reviews the results from a number of game experiments that establish a unitary system for forming subjective expected utility maps in the brain, and acting on these maps to produce choices. Social situations require the brain to build an understanding of the other person using neuronal mechanisms that share affective and intentional mental states. These systems allow subjects to better predict other players' choices, and allow them to modify their subjective utility maps to value pro-social strategies. New results for a trust game are presented, which show that the trust relationship includes systems common to both trusting and trustworthy behaviour, but they also show that the relative temporal positions of first and second players require computations unique to that role.     

Neural correlates of saccadic suppression in humans

When you look into a mirror and move your eyes left to right, you will see that you cannot observe your own eye movements. This demonstrates the phenomenon of saccadic suppression: during saccadic eye movements, visual sensitivity is much reduced. Given that humans make more than 100,000 eye movements each day, it is clear why suppression is needed: without it, the motion on the retina would prevent us from seeing anything at all. Psychophysical data show that suppression is stimulus selective: it is strongest for the kind of stimuli that preferentially activate magnocellular thalamic neurons. This has led to the hypothesis that saccadic suppression selectively targets the magnocellular stream. We used fMRI to find brain areas with a stimulus-selective suppression of the BOLD signal that matches the psychophysical data. We found such a neural correlate of saccadic suppression in the dorsal stream (hMT+, V7) and in ventral area V4. These areas receive magnocellular input; hence our findings are consistent with the magnocellular hypothesis. The range of effects in our data and in single cell data, however, argues against a single thalamic mechanism that suppresses all cortical input. Instead, we speculate that saccadic suppression relies on multiple mechanisms operating in different cortical areas.     

Neural correlates of chromatic motion perception.

A variety of psychophysical and neurophysiological studies suggest that chromatic motion perception in the primate brain may be performed outside the classical motion processing pathway. We addressed this provocative proposal directly by assessing the sensitivity of neurons in motion area MT to moving colored stimuli while simultaneously determining perceptual sensitivity in nonhuman primate observers. The results of these studies demonstrate a strong correspondence between neuronal and perceptual measures. Our findings testify that area MT is indeed a principal component of the neuronal substrate for color-based motion processing.     

Neural correlates of associative training in Hermissenda

Hair cells in Hermissenda respond to illumination of the ipsilateral and contralateral eyes. These responses are modified by associative training of the animal. The observed electrophysiological changes appear to result from changes in the photoreceptors' synaptic input to the hair cells.     

Polarization contrast and motion detection

Form and motion perception rely upon the visual system’s capacity to segment the visual scene based upon local differences in luminance or wavelength. It is not clear if polarization contrast is a sufficient basis for motion detection. Here we show that crayfish optomotor responses elicited by the motion of images derived from spatiotemporal variations in e-vector angles are comparable to contrast-elicited responses. Response magnitude increases with the difference in e-vector angles in adjacent segments of the scene and with the degree of polarization but the response is relatively insensitive to the absolute values of e-vector angles that compose the stimulus. The results indicate that polarization contrast can support visual motion detection.