Onset Of Visual Stimulus Research Articles

ON- and OFF-pathways form separate neural substrates for motion perception: psychophysical evidence

We have tested the hypothesis that in humans the signals carried by ON- and OFF-pathway respectively are processed for the perception of motion by two distinct physiological substrates. In vertebrates, onset of a bright visual stimulus is signaled to the CNS by ON-center retinal ganglion cells; onset of a dark stimulus is transmitted by OFF-center cells. We chose apparent motion generated by successive presentation of two adjacent lines as a stimulus. Lines presented on a bright background were either darker or brighter than this background. Delayed onset of a pair of bright or dark lines elicits apparent motion at the same time fulfilling the constraint of stimulating either ON- or OFF-center ganglion cells, respectively. We determined the threshold delay needed for subjects to perceive the temporal order of the onset of the two lines for various angular separations. The threshold delay for a pair of bright lines stayed low for separations from 2' to 7'. The threshold delay for a pair of dark lines was low only within a narrow range of separations centered around 3'. The variation of thresholds with line distance must reflect the existence of a limited processing zone for the perception of motion. The diameter of the processing zone for bright lines is about twice as large as that for dark lines. This suggests that in humans the separation of ON- and OFF-pathways extends to the early stages of motion perception. To test this hypothesis independently, thresholds were determined when a bright and a dark line were presented in succession. This was done for a separation of 3' where thresholds for a pair of lines with equal contrast are similarly low. Temporal order was perceived correctly only when the delay was at least two to four times as high as the threshold delays found for the equal contrast stimuli.

Role of the rostral superior colliculus in active visual fixation and execution of express saccades.

1. In the rostral pole of the monkey superior colliculus (SC) a subset of neurons (fixation cells) discharge tonically when a monkey actively fixates a target spot and pause during the execution of saccadic eye movements. 2. To test whether these fixation cells are necessary for the control of visual fixation and saccade suppression, we artificially inhibited them with a local injection of muscimol, an agonist of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). After injection of muscimol into the rostral pole of one SC, the monkey was less able to suppress the initiation of saccades. Many unwanted visually guided saccades were initiated less than 100 ms after onset of a peripheral visual stimulus and therefore fell into the range of express saccades. 3. We propose that fixation cells in the rostral SC form part of a fixation system that facilitates active visual fixation and suppresses the initiation of unwanted saccadic eye movements. Express saccades can only occur when activity in this fixation system is reduced.

Basal ganglia motor control. II. Late pallidal timing relative to movement onset and inconsistent pallidal coding of movement parameters

1. We have tested the hypothesis that the basal ganglia initiate some one or several modes of movement by recording the change in discharge frequency of pallidal neurons during visually triggered step and visually paced ramp moves in relation to the visual stimulus onset, the change in the electromyograph (EMG), and the movement onset of trained rhesus monkeys. 2. The modal times of change for globus pallidus pars interna (GPi) were significantly later than those for forearm agonist muscle EMG. By contrast, the modal time of change for the cerebellar dentate nucleus preceded that for wrist agonist EMG. 3. The direction of change in discharge frequency of the GPi cells was for 71% an increase and for 29% a decrease. 4. Because of the relatively late change of activity of GPi neurons, we propose that GPi neurons cannot initiate these movements, as others have also suggested. The commands for the initiation of these movements may instead be generated by structures that include the lateral cerebellum and the anterior cerebral cortex. 5. We have also tested the hypothesis that the pallidum of the basal ganglia or the dentate of the lateral cerebellum may control the direction and other parameters of the trajectory by recording from both structures to see whether cell discharge correlated with the parameter and whether the correlation was consistent across tasks. Two rhesus monkeys were trained to perform hold-ramp-hold and hold-step-hold visually guided movements in opposite directions by flexing and extending the wrist with and against uniform oppositely directed torque loads (0.2 Nm). Wrist position, velocity, force, and EMG were recorded simultaneously. Movement amplitudes and directional intent were computed and inferred, respectively. 6. Task related neurons were classified as follows: 1) directional, if the discharge rate was reciprocal for opposite movements or if it increased or decreased during movement in one direction only; 2) bidirectional, if the discharge rate increased or decreased during movement in both directions; and 3) "other," if it was directional under one load and bidirectional under the other. During step tracking, 34 GPi, 47 globus pallidus pars externa (GPe), and 44 cerebellar dentate nuclear neurons were related to the task. Of the GPi cells, 14 (41%) were directional, 6 (18%) bidirectional, and 14 (41%) other. Of the GPe neurons, 13 (28%) were directional, 19 (40%) bidirectional, and 15 (32%) other. Of the dentate cerebellar nuclear cells, 5 (11%) were bidirectional, 31 (70%) bidirectional, and 8 (18%) other.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of passive eye movement on retinogeniculate transmission in the cat

1. The nature and time window of interaction between passive phasic eye movement signals and visual stimuli were studied for dorsal lateral geniculate nucleus (LGNd) neurons in the cat. Extracellular recordings were made from single neurons in layer A of the left LGNd of anesthetized paralyzed cats in response to a normalized visual stimulus presented to the right eye at each of several times of movement of the left eye. The left eye was moved passively at a fixed amplitude and velocity while varying the movement onset time with respect to the visual stimulus onset in a randomized and interleaved fashion. Visual stimuli consisted of square-wave modulated circular spots of appropriate contrast, sign, and size to elicit an optimal excitatory response when placed in the neurons' receptive-field (RF) center. 2. Interactions were analyzed for 78 neurons (33 X-neurons, 43 Y-neurons, and 2 physiologically unclassified neurons) on 25-65 trials of identical visual stimuli for each of eight times of eye movement. 3. Sixty percent (47/78) of the neurons tested had a significant eye movement effect (ANOVA, P less than 0.05) on some aspect of their visual response. Of these 47 neurons, 42 (89%) had a significant (P less than 0.05) effect of an appropriately timed eye movement on the number of action potentials, 36 (77%) had a significant effect on the mean peak firing rate, and 31 (66%) were significantly affected as evaluated by both criteria. 4. The eye movement effect on the neurons' visual responses was primarily facilitatory. Facilitation was observed for 37 (79%) of the affected neurons. For 25 of these 37 neurons (68%), the facilitation was significant (P less than 0.05) as evaluated by both criteria (number of action potentials and mean peak firing rate). Ten (21%) of the affected neurons had their visual response significantly inhibited (P less than 0.05). 5. Sixty percent (46/78) of the neurons were tested for the effect of eye movement on both visually elicited activity (visual stimulus contrast = 2 times threshold) and spontaneous activity (contrast = 0). Eye movement significantly affected the visual response of 23 (50%) of these neurons. However, spontaneous activity was significantly affected for only nine (20%) of these neurons. The interaction of the eye movement and visual signals was nonlinear. 6. Nine of 12 neurons (75%) tested had a directionally selective effect of eye movement on the visual response, with most (8/9) preferring the temporal ward direction.(ABSTRACT TRUNCATED AT 400 WORDS)

Suppression of visually initiated hand movement by stimulation of the prefrontal cortex in the monkey

Five adult monkeys ( Macaca fuscata) were trained for the go/no-go hand movement task with discrimination between two different color stimuli. The go stimulus was accompanied by a reward when a monkey lifted a lever by wrist extension within the stimulus duration (500 ms), whereas the no-go stimulus was not. The monkey revealed the potential specific to the no-go response in the prefrontal cortex, called ‘no-go potential’, i.e. surface-negative, depth-positive deflexions in the cortex of the dorsal bank of the principal sulcus and of the rostroventral corner of the prefrontal region. Effects of electrical stimulation of these prefrontal areas upon the go response were observed and analyzed in order to study functional significances of the no-go potential. The surface and depth (2.0–3.0 mm) electrodes chronically implanted respectively in various cortical areas of both hemispheres, originally used for recording cortical field potentials, were utilized for bipolar stimulation of the cortical area. A train of brief electrical pulses was delivered to the loci producible of the no-go potential at different times after the onset of go visual stimulus. The stimulation suppressed the go movement by cancelling and delaying. The grade of the suppressor effect depended on the timing of electrical stimulation after the onset of visual stimulus, and was maximal at around the time of appearance of the no-go potential. The suppressor effects was compared with that produced by stimulating some other areas in the prefrontal cortex and the premotor cortex, and was found rather unique in those areas producible of the no-go potential. The present study suggests that the no-go potential represents the suppressor action of the prefrontal areas by revoking and deferring the motor command initiated by the visual stimulus.

Activity of trigeminothalamic neurons in medullary dorsal horn of awake monkeys trained in a thermal discrimination task.

We analyzed the activity of 51 trigeminothalamic neurons in the medullary dorsal horn (trigeminal nucleus caudalis) of monkeys during the performance of behavioral tasks requiring the monkeys to discriminate innocuous and noxious thermal stimuli applied to the face and to detect the onset of visual stimuli. Static properties of trigeminothalamic neurons in behaving monkeys were similar to those in anesthetized monkeys. Responses to passively presented mechanical and thermal stimuli, receptive-field properties, and conduction velocities did not differ in the awake and anesthetized states. For most wide dynamic range and nociceptive-specific trigeminothalamic neurons, there was a negative correlation between the magnitude of thermally evoked activity and behavioral latencies to discriminate 47 and 49 degrees C stimuli. Thus, both groups of neurons provide information that could be used by the monkey to discriminate noxious thermal stimuli. The magnitude of thermal responses of trigeminothalamic neurons was modulated by the behavioral significance of the stimulus. Behaviorally relevant thermal stimuli presented during the thermal discrimination task produced a greater neuronal response than equivalent irrelevant thermal stimuli presented between behavioral trials or presented while the monkey performed the visual detection task. Neurons whose activity is modulated by behavioral state are likely to be involved in discrimination of thermal stimuli, since the activity of these neurons correlates with the behavioral response to the stimuli and information from the modulated neurons is sent to the thalamus. Some trigeminothalamic neurons that exhibited somatosensory responses also responded to behaviorally relevant stimuli and events associated with trial initiation and receipt of reward in the behavioral tasks. Similar events outside a behavioral task evoked no neuronal responses. These task-related responses were similar to those described previously for medullary dorsal horn neurons not identified as to projection sites (14).(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of increasing age on the latency for saccadic eye movements.

The latency for saccadic eye movements to a visual stimulus was studied in 59 adults whose ages ranged from 20.7 to 79.5 years. All were free of neurologic disease and drug use. Horizontal eye movements were recorded by electrooculography and the latency from the onset of a peripheral visual stimulus to the onset of a saccadic refixation eye movement was determined. A linear regression analysis revealed a correlation between increasing age and increasing latency for saccadic refixation eye movements. The direct relationship between increasing age and increasing latency for saccadic eye movements is a factor that should be taken into account in eye-movement studies as well as other methodologies such as tachistoscopic studies in which saccadic eye movements play a role in study design.

Temporal order detection for foveal and peripheral visual stimuli

A measure of the temporal resolution of the visual system is the detection of temporal order of the onset of adjacent visual stimuli. The threshold is a few milliseconds and its variation with stimulus separation suggests the existence of a limited processing zone. Measurements were obtained of this distance function in the fovea and retinal periphery of two observers. Temporal order detection threshold does not vary greatly with retinal position out to 20 deg eccentricity, nor does the distance function change much in width, except near the fovea. An independent estimate of the zone for this kind of processing was obtained by testing the range within which signals from oppositely-directed stimulus pairs cancel. Thresholds for dichoptic stimuli are much higher than for monocular or binocular ones.

Saccade latency and warning signals: Effects of auditory and visual stimulus onset and offset

Previous studies have found that a nonspecific visual event occurring at the fovea 50–150 msec after the onset of a peripheral target delayed the initiation of the saccade to that target. The present studies replicated and extended this finding by studying the effects of both visual and auditory warning signals, by examining the effects of onset and offset warning on manual response latency, and by investigating the effects of presenting the warning events in the periphery of the visual field. The results indicated that the interfering effects occur with visual but not auditory stimuli, with saccades but not motor responses, and when the visual warning event occurs either foveally or in the subject’s periphery. Implications for the processes involved are discussed.

CS–US interval as a determinant of the form of Pavlovian appetitive conditioned responses.

The effects of variation in temporal and predictive relations between the onset of visual and auditory conditioned stimuli (CSs) and delivery of a food unconditioned stimulus (US) on the form of conditioned responses (CRs) were examined in four experiments with rat subjects. In Experiment 1, substantially different CRs were evoked by CSs differing in duration and CS-US interval in delay conditioning. Using a second-order conditioning technique to assess the associative strength of CSs of different delay intervals, Experiment 2 showed that shorter intervals generated more strength than longer intervals, but the differences in CR form observed in Experiment 1 were not dependent on differences in associative strength. In Experiment 3, CSs of equal duration but different CS-US intervals in trace conditioning evoked CRs similar in form to those observed with delay procedures in Experiment 1. In Experiment 4, the onset of a CS of intermediate CS-US interval was made less predictive of US delivery by separate nonreinforced short duration presentations of that stimulus. Behavior evoked by the intermediate interval CS was more like that evoked by CSs of longer CS-US intervals. These data are discussed in terms of an orienting response hypothesis proposed earlier to account for differences in the nature of CRs evoked by CSs differing in physical characteristics such as modality.

Cortical responses from adults and infants to complex visual stimuli

Event-related potentials (ERPs) time-locked to the onset of visual stimuli were extracted from the EEG of normal adult (N = 16) and infant (N = 23) subjects. Subjects were not required to make any response. Stimuli delivered to the adults were 150 msec exposures of 2 sets of colored slides projected in 4 blocks, 2 in focus and 2 out of focus. Infants received 2-sec exposures of slides showing people, colored drawings or scences from Disneyland, as well as 2-sec illuminations of the experimenter as she played a game or of a TV screen the baby was watching. The adult ERPs showed 6 waves (N1 through P4) in the 140–600-msec range; this included a positive wave at around 350 msec that was large when the stimuli were focused and smaller when they were not. The waves in the 150–200-msec range, by contrast, steadily dropped in amplitude as the experiment progressed. The infant ERPs differed greatly from the adult ones in morphology, usually showing a positive (latency about 200 msec)—negative (5–600 msec)—positive (1000 msec) sequence. This ERP appeared in all the stimulus conditions; its presence or absence, furthermore, was correlated with whether or not the baby seemed interested in the stimuli. Four infants failed to produce these ERPs; an independent measure of attention to the stimuli, heart rate deceleration, was demonstrated in two of them. An electrode placed beneath the eye to monitor eye movements yielded ERPs closely resembling those derived from the scalp in most subjects; reasons are given for assigning this response to activity in the brain, probably at the frontal pole. This study appears to be one of the first to search for cognitive ‘late waves’ in a no-task situation. The results suggest that further work with such task-free paradigms may yield additional useful techniques for studying the ERP.

PERIPHERAL VISUAL RESPONSE TIME AND RETINAL LUMINANCE-AREA RELATIONS*

Button pressing response time was measured in five trained subjects to the onset of small visual stimuli imaged across the horizontal retinal meridian at 10 degrees arc intervals from 90 degrees arc right to 90 degrees arc left of the line of sight. Four stimulus luminance conditions were presented at each peripheral retinal location, a low luminance condition (approx. 2.6 ft-L), a high luminance condition (arox. 22.2 ft-L), and two conditions in which luminance was varied systematically from low to moderately intense levels from one side of the visual field to the other. The two variable luminance conditions were included to see if it is possible to compensate for the progressive decrease in retinal stimulus image area caused by the apparent pupil. The results indicated that response time to stimuli imaged within approximately 70 degrees arc from the fovea does not differ significantly when all possess about the same luminance. Mean response time is significantly faster to stimuli imaged beyond about 70 degrees arc from the fovea when their luminance is increased by an amount equal to the foveal stimulus luminance multiplied by the cosine of the angle between the peripheral stimuli and the line of sight. These data are discussed in relation to previous psychophysical data and to possible response mechanisms.

Reaction time to onset and cessation of a visual stimulus

An experiment was conducted to determine if differences exist in reaction time to onset and cessation of a large, bright, foveal light. Out of 21 replications of the experiment, eight differences, split equally between the two types of reactions, were significant. These findings are attributed to the trained subjects’ sensitivity to momentary fluctuations in their physiological and psychological states. It is concluded that no differences between the two types of reaction are apparent at these parameters.