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At least 1,063 records · Page 59Linked to original sources

PubMed2006

Visual resolution with retinal implants estimated from recordings in cat visual cortex.

We investigated cortical responses to electrical stimulation of the retina using epi- and sub-retinal electrodes of 20-100 microm diameter. Temporal and spatial resolutions were assessed by recordings from the visual cortex with arrays of microelectrodes and optical imaging. The estimated resolutions were approximately 40 ms and approximately 1 degrees of visual angle. This temporal resolution of 25 frames per second and spatial resolution of about 0.8 cm at about 1m and correspondingly 8 cm at 10 m distance seems sufficient for useful object recognition and visuo-motor behavior in many in- and out-door situations of daily life.

Action Potentials↗

PubMed1992

Visual experience, visual field size, and the development of nonvisual sensitivity to the spatial structure of outdoor neighborhoods explored by walking.

When places are explored without vision, observers go from temporally sequenced, circuitous inputs available along walks to knowledge of spatial structure (i.e., straight-line distances and directions characterizing the simultaneous arrangement of the objects passed along the way). Studies show that a life history of vision helps develop nonvisual sensitivity, but they are unspecific on the formative experiences or the underlying processes. This study compared judgments of straight-line distances and directions among landmarks in a familiar area of town by partially sighted persons who varied in types and ages of visual impairment. Those with early childhood loss of broad-field vision and those blind from birth performed significantly worse than those with early or late acuity loss and those with late field loss. Broad-field visual experience facilitates perceptual development by providing a basis for proprioceptive and efferent information from locomotion against distances and directions relative to the surrounding environment. Differences in the perception of walking, in turn, cause the observed differences in sensitivity to spatial structure.

Adolescent↗

PubMed1989

Visual discrimination by cats given lesions of visual cortex in one or two stages in infancy or in one stage in adulthood.

Sparing of visual function was studied in cats with bilateral cortical damage to Areas 17 and 18 and most of Area 19. Cats with lesions made in 2 stages, on Postnatal (P) Days 3 and 6, in 1 stage on P6, or in 1 stage in adulthood were compared with sham-operated controls on 10 visual discrimination tasks. On some tasks, both groups of cats that underwent surgery as infants showed considerable sparing of function compared with cats that had surgery as adults; the latter group showed a marked impairment. However, on several of the discriminations, 2-stage lesions permitted almost total sparing of pattern vision, whereas 1-stage lesions made neonatally were almost as debilitating as those incurred in adulthood. The findings suggest that differential behavioral consequences can follow physiological or anatomical changes, or both, that occur within a 4-day neonatal interoperative period.

Aging↗

PubMed1987

Functional development of the visual system in normal and protein-deprived rats. IV. Latencies in the specific visual pathway of adult rats.

It was recently shown that an increase in latency of the cortical visual evoked response (VER) seen in young malnourished animals persists in adult rats given a low protein diet. In the present paper the VER latencies of the specific visual pathway were measured in order to establish the level at which the latency increase occurs in protein-deprived rats. The VER in the dorsal lateral geniculate nucleus (dLGN) showed no significant differences to onset or peak latencies between control (C) and the protein-deprived (PD) rats. The dLGN activity was higher in C rats than in PD rats. Late components differed in median values between the two groups, but the individual variations were large. Generally, the VER of the dLGN in PD rats was more stereotype compared with the C rats. Intracortical VER at a depth of 0.4-0.5 mm showed a small negative component of short onset latency in both C and PD rats. This component preceded the onset of the initial positivity recorded from the cortical surface by 1-2 ms in C rats and by 3-6 ms in PD rats. Following electrical stimulation of the dLGN, no differences in onset latency of the first cortical activity (monosynaptic response) were recorded between C and PD rats, whereas later activity was significantly delayed in PD compared with C rats. The laminar potential pattern was less distinct in PD compared with C rats, and the late components of the evoked response from deep cortical layers were markedly attenuated or lacking.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗

PubMed1995

Visual evoked potentials generator model derived from different spatial frequency stimuli of visual field regions and magnetic resonance imaging coordinates of V1, V2, V3 areas in man.

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli were recorded from 24 scalp derivations (including zygomatic and inion) referenced to digitally linked earlobes in 50 controls. 1, 2 and 4 cpd patterns were presented as full field (FF) stimuli, on Upper Hemifields (UHF) and Lower Hemifields (LHF), upper and lower quadrants and with the occlusion of central and peripheral UHF and LHF. VEPs to octant stimuli were also recorded with 2 cpd patterns. N1, P1 and N2 components were recorded from posterior and inion derivations with FF stimuli, from posterior derivations with LHF stimuli, only from inion leads with UHF stimuli, from derivations ipsilateral to stimuli with quadrants and octants, and consistently from midline derivations only with lower quadrants. Polarity inverted sequences (iN1-iP1-iN2) were recorded from the other scalp derivations, with similar latency and spatial frequency sensitivity as N1-P1-N2. Single Equivalent Dipole (ED) calculations were performed on N1 and P1 recorded in the different stimulus conditions. Our findings contradict previous hypotheses on VEP generators and contradict the predictions of VEPs polarity and distribution based on the "cruciform model" of VEPs generators. In order to explain the distribution of VEPs to upper and lower half fields and to quadrant and octants, we propose a model based on the position of the medial and occipito-polar surface of visual cortex in man.

Adult↗

PubMedDate not supplied

[The phases of visual cortex and hippocampal evoked potentials in rabbit reflect the orienting reaction in case of visual stimuli intensity changes].

The oddball paradigm was applied in experiments with waking rabbits using rare (deviant) and frequent (standard) stimuli, which were similar in color but different in intensity, deviant ones being of lower intensity. In addition, the VEPs to the single deviants were averaged. Such single deviants (without standard stimuli) were given at the beginning and at the end of the stimulation. The positivity of cortical and hippocampal visual evoked potentials to deviant stimuli increased in comparison to responses to standards and single deviants. The VEP-peaks P1 and P2 in the visual cortex and the VEP P1, N1 and P2 peaks in hippocampus increased. The most prominent significant changes were demonstrated for cortical VEP P2 (P130) peak. It is suggested that the increase of positivity to oddball deviants is due to the orienting reflex arising in response to rare stimuli. The increase of P2-peak can be connected with the transfer of information signaling changes of light intensity. It was demonstrated that the most clear and contrast differences in the VEPs to deviants and standards took place for the case of minimal distinction in their intensities. This fact may reflect the orienting reactionto threshold stimuli.

Animals↗

PubMed1987

Visual acuity prediction using the visual evoked response.

A technique is described for predicting visual acuity in adult humans by using the visually evoked response (VER). This involves using a pattern-reversal checkerboard with a small check size (5.5. min arc) as the stimulus, and measuring the peak-to-trough amplitude of the VER wave arising from it. The technique is therefore both rapid and convenient. The pattern-reversal rate was varied in order to assess the influence of the temporal modulation on the VER waves. The results suggest that a rapid pattern reversal rate produces a VER which displays a reduced association with spatial processing.

Adult↗

PubMed1984

Critical period plasticity for visual function: definition in monocularly deprived rats using visually evoked potentials.

VEPs were used to study the effects of visual deprivation and to define the critical period in the hooded rat. It is shown that monocular deprivation in young animals reduces both the visual acuity and the contrast sensitivity of the deprived eye and that the depth of attenuation depends upon the age at onset of deprivation. This suggests that the critical period is almost over by 6 weeks postnatally. This simple recording technique might be useful in studying amblyopic children and in defining more clearly the critical period for human vision.

Aging↗

PubMed1995

Projections from the 17/18 visual area transition zone to the ipsilateral visual field representation in the cat superior colliculus.

We demonstrate by retrograde tracing methods that the transition zone between visual cortical areas 17 and 18--where a representation of the ipsilateral visual field (IVF) has been described--projects to the IVF representation in the ipsilateral superior colliculus. The stratum griseum superficiale is the target of this retinotopically organized cortical projection to the rostral part of the superior colliculus.

Animals↗

PubMed1996

[Objective determination of visual acuity. Improvement of an infrared nystagmography method and comparison with pattern visual evoked potentials].

BACKGROUND: The purpose of this study was to optimize a new nystagmographical method of objective assessment of visual acuity and compare it to an electrophysiological method. METHODS: In the nystagmographical method motionless brake marks were superimposed on a moving grating pattern, causing suppression of optokinetic nystagmus (OKN). The smallest brake mark that interrupted the OKN was taken as an objective measure for visual acuity (VA). In the electrophysiological method rapid sequences of high-contrast checkerboard patterns of different spatial frequency were presented in a pattern on-off mode in a foveal field. The VEP response function was extrapolated to 0 microV. The objective values of 94 healthy eyes with artificially reduced vision (VA 0.05-2.0) and 314 eyes with pathological reduction of VA due to different etiologies were compared. RESULTS. The selectivity of the nystagmographical method could be improved. With the exception of strabismic amblyopia, the results were closely correlated with the VA. The least VA could be estimated in steps of 0.06, 0.1, 0.2, 0.5 and 0.8. CONCLUSIONS: The new nystagmographical method yields useful information about the least VA if strabismic amblyopia is excluded.

Adolescent↗

PubMed1998

[Visual evoked potentials and static perimetry in functional visual deficits].

The author presents application of visual evoked potentials and static perimetry in hysterical amblyopia, psychical blindness, malingerers, amblyopia, cortical blindness and visual acuity assessment in infants. The author emphasizes the influence of multiple factors on the results and interpretation of the examination.

Evoked Potentials, Visual↗

PubMed1985

Post-suppression vestibulo-ocular reflex in man: visual and non-visual mechanisms.

The vestibulo-ocular reflex (VOR) was intermittently suppressed by fixating a head-fixed target (light on: 9 s; darkness: 2 s) during whole-body (60-100 deg/s peak; 0.01-0.17 Hz) sinusoidal and triangular oscillation about the vertical axis for about 2 min. Eye movements associated with intervals of darkness at different phases of body oscillation were stimulus-locked averaged, thereby estimating the "post-suppression VOR". The gain ( (eye velocity)/(head velocity) ) of the post-suppression VOR was generally 26-50% of the normal gain in maintained darkness. During sinusoidal body oscillation, the phase lead of the post-suppression VOR equalled or, at low frequencies, appeared to exceed the normal lead in maintained darkness. When the light was permanently extinguished after 2 min of intermittent suppression, an initially reduced VOR rapidly (e.g., 0.4 s) appeared and required 6-30 s to build up to normal again. These observations indicated that visual suppression of the VOR dissipated in darkness with rapid and slow components. During 0.17 Hz, 60 deg/s sinusoidal oscillation, the rapid (perhaps visual) component was responsible for 1/3, and the slow (perhaps non-visual) component for 2/3, of all suppressive effects.

Adaptation, Ocular↗

PubMed2006

Adaptation to visual feedback delays in manual tracking: evidence against the Smith Predictor model of human visually guided action.

We report adaptation to delayed visual feedback during a manual tracking task, testing the nature of the adapted responses with frequency analysis. Two groups of seven subjects tracked unpredictable targets using a handheld joystick, alternating between pursuit and compensatory display trials. The test group then practised for 1 h per day with a visual feedback delay of 300 ms; the control group practice under normal undelayed conditions. Introduction of the visual feedback delay significantly disrupted tracking performance, with an increase in errors and a reduction in frequency of corrective movements. Subjects showed clear evidence of adaptation during the 5 day experiment, decreasing tracking error and decreasing the mean power of intermittent corrections. However, there was no evidence of a return towards the initial high frequency intermittent tracking. We suggest that the adaptation observed in this study reflects the modification of predictive feedforward actions, but that these data do not support control based on Smith Prediction.

Adaptation, Physiological↗

PubMed1997

Dopaminergic modulation of visual responses in toads. I. Apomorphine-induced effects on visually directed appetitive and consummatory prey-catching behavior.

This study confirms for a phylogenetically basal terrestrial vertebrate that dopaminergic modulations interfere with the visually directed appetitive and consummatory feeding behaviors orienting and snapping, respectively. (1) In common toads Bufo bufo, intralymphatic administration of the dopamine D2/D1-receptor agonist apomorphine led to a dose-dependent facilitation of prey-snapping in response to moving objects. The snapping activity reached a maximum 15-35 min after apomorphine injection. (2) To changes in configurational stimulus features, the basic pattern of discrimination was maintained; however, the acuity of discrimination was reduced due to the high snapping response level. (3) The apomorphine-induced facilitation of snapping was accompanied by a suppression of prey-oriented lunging and turning. Toads snapped only if prey occurred frontally in the visual field at a relatively short distance. The snapping behavior was fixed in its form and stereotyped regarding its immediate release. (4) About 90 min after apomorphine administration, prey-oriented turning behavior was restored and displayed a facilitatory rebound. (5) In comparative experiments with the species B. marinus, both prey-oriented turning and snapping responses were suppressed by apomorphine in a dose-dependent manner. (6) After pre-treatment with the dopamine antagonist haloperidol, apomorphine showed no measurable effect on the visual release of prey orienting or snapping. (7) The results contribute to the sensorimotor and the motivation hypothesis of dopamine function proposed for higher vertebrates and stimulate a comparative discussion of anatomic homologies and functional analogies.

Animals↗

PubMed1986

The accuracy of aimed movements to visual targets during development: the role of visual information.

Children aged 1.5 to 8 years were required to touch accurately an illuminated target lamp located on a vertical board. Movements were made when visual information was complete (target lit for 3 s, room illuminated; partial (target lit for 3 s, room dark, and reduced (target lit for 0.7 s, room dark). Dependent variables were response accuracy, reaction time, and movement time. Accuracy decreased with decreasing availability of visual information and improved with age under all conditions. Reaction times were shorter in the dark (Conditions 2 and 3) than in the light; they decreased with age up to age 5 and did not continue to decrease thereafter. Movement time did not change with age under Conditions 1 and 3 but tended to increase with age under Condition 2. Slower movements were more accurate at all ages, provided visual feedback could be utilized. Increased reliance on the strategy "slower movements yield higher accuracy" was held to account for developmental changes under Condition 2, whereas in Conditions 1 and 3 improvement in the efficiency of motor preprogramming was implicated.

Age Factors↗

PubMed1984

Visual pigments and the labile scotopic visual system of fish.

Among mammals, birds, most reptiles and chondrichthians, only rhodopsins are present. Among agnathans, osteichthians, amphibians and certain freshwater turtles there are species having only porphyropsins or only rhodopsins or, more interestingly, both pigments, either sequentially or together. This latter grouping represents the paired-pigment species. Associated with the presence of paired-pigments is the possibility that the proportions of rhodopsin and porphyropsin may change. Depending on the characteristics of each paired-pigment species, naturally occurring changes in visual pigment ratios are related to migrations in anadromous and catadromous teleosts and anadromous cyclostomes and to seasonal variation in several teleosts. In addition, the visual pigment composition of certain species of teleosts has been altered by the specific effects of light, temperature, diet and hormones. Of two possible mechanisms for altering spectral sensitivity, varying the proportion of rhodopsin and porphyropsin is far more common than utilizing a single chromophore and changing the opsin. In addition to the long established evidence that extractable rod pigment ratios may change during the life cycle or in response to specific exogenous factors, there is the more recent recognition from microspectrophotometry that cone pigment ratios may also change in concert. The effect of lighting conditions and temperature on the visual pigment composition of certain paired-pigment species is presented.

Animals↗

PubMed2006

Discrete bandwidth visual feedback increases structure of output as compared to continuous visual feedback in isometric force control tasks.

BACKGROUND: Performance variability measures provide a partial picture of force control ability. Nonlinear analyses can reveal important information related to the randomness and complexity of the data, providing a more complete picture of the physiological process. METHODS: We investigated the effects of visual feedback on the structure and performance of the force output from isometric force control tasks. Twelve young volunteers completed isometric force control tasks using two types of visual feedback: discrete bandwidth (+/-4% maximal voluntary contraction) and continuous line matching. We determined force signal variability (standard deviation), self-similarity (fractal dimension), and complexity (approximate entropy). Analyses of variance (feedback x muscle group x force level) were conducted and P values less than 0.05 were considered significant. FINDINGS: The force signal in discrete bandwidth feedback, compared to continuous line matching, had significantly a higher standard deviation (P=.000): 2.18 Nm (SD 1.98) vs. 0.99 Nm (SD 0.91); lower fractal dimension (P=.000): 1.07 (SD 0.04) vs. 1.16 (SD 0.04); and lower approximate entropy (P=.000): 0.12 (SD 0.07) vs. 0.26 (SD 0.09). INTERPRETATION: The greater self-similarity (lower fractal dimension) and greater regularity (lower approximate entropy) of the discrete bandwidth, compared to the continuous line matching, may indicate a process that required more kinesthetic (intrinsic) feedback to modulate force. Clinicians may choose to employ visual feedback paradigms that target the use of intrinsic feedback during rehabilitation. Discrete bandwidth feedback may be useful for delineating impairments in motor skill and measuring outcomes of intervention programs.

Adult↗

PubMed2006

Two-stroke: a new illusion of visual motion based on the time course of neural responses in the human visual system.

A sequence of static images presented in rapid succession can create a powerful impression of visual movement, a fact exploited by the visual media (television and cinema) and by animators. A new illusion of movement called "two-stroke" is described, in which repeated presentation of a two-frame pattern displacement can create an impression of continuous forward motion, without the inclusion of any additional pattern displacements. The illusion can be explained by a biphasic temporal impulse response that modifies the stimulus delivered to motion energy sensors. It offers a basis for further research on temporal and motion responses in the visual system as well as a tool for animators and graphic artists to create consistent apparent movement from minimal external stimulation.

Humans↗