Curvature detection in the visual field and a possible physiological correlate

Summary Discrimination of curvature in nearly straight stimuli was measured for twelve stimulus orientations, corresponding to the hours of the clock, at eleven positions in the visual field of human observers. Displacements below the diameter of a photoreceptor can be detected foveally. In the periphery of the visual field, stimuli oriented towards the fovea yield best results. This anisotropy of orientation discrimination seems not to have an optical cause. It has a close correlation both to the structure and functional properties of cat ganglion cells, to the organization of receptive fields in area 17 and to the geometry of ocular dominance stripes.     

Orientation deprivation in cat: What produces the abnormal cells?

Summary When an animal is reared with goggles containing stripes of a single orientation, many cells in the visual cortex become abnormal. To find out whether these abnormal cells result from orientation deprivation per se or from other features of goggle rearing we reared kittens with goggles containing stripes of many orientations. The data suggest that orientation deprivation contributes to the development of abnormal cells but is not its sole cause.This work was supported by grant EY00125 from the National Eye Institute. Joelle Presson was supported by the Systems Physiology Training grant 5T326M07257     

Cross-modal links between vision and touch in tactile discrimination of line orientation

This study examined visuotactile cross-modal interactions in the identification of tactile stimuli. In Experiment 1, participants were required to discriminate the orientation of tactile target stimuli (line segments with left and right diagonal orientations) presented at the tip of the participant's left index finger via an Optacon, while trying to ignore a task-irrelevant visual diagonal line presented with a 200-ms or 700-ms stimulus onset asynchrony (SOA). The results show that the participants responded faster when the orientation of the tactile stimuli were congruent with the orientation of the visual stimuli than when they were incongruent in the 200-ms SOA condition. To assess at what level of processing the reaction time advantage obtained with congruent orientation trials takes place, Experiment 2 served as a replication of Experiment 1 with somatosensory event-related brain potential (ERP) recordings. ERP effects of orientation congruency were observed between 200 and 350 ms post-stimulus, suggesting that visuotactile cross-modal interactions may occur at intermediate perceptual-cognitive stages rather than at early sensory-specific or late motor-related stages.     

State dependent activity in monkey visual cortex

Responses were recorded from isolated neurons in the visual cortex of rhesus monkeys while they performed an orientation match to sample task. In each trial the animal was first cued with randomly selected orientation, and then presented with a sequence of gratings whose orientations were randomly selected. The animal was required to release a switch when it saw a grating that matched the cued orientation. For some recordings the animal was given a tactile cue by having it feel the orientation of a grooved plate that it could not see. In other experiments the cue orientation was presented visually on the screen in front of the animal and then removed before the sequence of gratings was presented. Using this task it was possible to determine if a neuron's response to a particular orientation was affected by whether or not it was the orientation for which the animal was looking. Over half the neurons examined in V4 (110/192) responded differently to the visual stimuli when the animal was cued to look for different orientations. For some neurons responses to all stimuli were strong when the animal was cued to look for a particular orientation, but weak when the same stimuli were presented in trials where the animal had been cued to look for another orientation. This type of sensitivity was found in neurons recorded while the animal was given a tactile cue, and also in other neurons tested when a visual cue was used, suggesting that the activity was not of direct sensory origin. In support of this, neurons in V4 were not strongly affected when the animal felt the grooved plate while not performing the orientation matching task. The prevalence of behavioral effects that was found using the orientation matching task suggests that extraretinal signals represent a prominent component of the activity in V4 of the behaving monkey.     

Poke and pop: Tactile–visual synchrony increases visual saliency

The majority of studies investigating interactions between vision and touch have typically explored single events, presenting one object at a time. The present study investigates how tactile–visual interactions affect competition between multiple visual objects in more dynamic cluttered environments. Participants searched for a horizontal or vertical line segment among distractor line segments of various orientations, all continuously changing color. Search times and search slopes were substantially reduced when the target color change was accompanied by a tactile signal. These benefits were observed even though the tactile signal was uninformative about the location, orientation, or color of the visual target. We conclude that tactile–visual synchrony guides attention in multiple object environments by increasing the saliency of the visual event.     

Persistence of visual-tactile enhancement in humans

We report two experiments in which non-informative vision of the finger enhanced tactile acuity on the fingertip. The right index finger was passively lifted to contact a grating. Twelve participants judged orientations of tactile gratings while viewing either the fingertip, or a neutral object presented via a mirror at the fingertip's location. In Expt. 1, tactile orientation discrimination for near-threshold gratings was improved when viewing the fingertip, compared to viewing the neutral object. Experiment 2 examined the temporal persistence of this effect, and found significant visual-tactile enhancement when a dark interval of up to 10 s intervened between viewing the finger and tactile stimulation. These results suggest that viewing the body modulates the neural circuitry of primary somatosensory cortex, outlasting visual inputs.     

Early selective visual experience and pattern discrimination in hooded rats.

The early visual experience of hooded rats was restricted to either vertical or horizontal stripes. In a discrimination task pairing a gray surface and stripes of either the same orientation or an orientation orthogonal to that experienced during rearing, the rats made significantly fewer correct choices with the orthogonal orientation. However, the relatively lower overall performance of the vertically reared-horizontally tested animals was a major factor in the main effect of testing condition. We conclude that functional of the rat visual system through early selective visual experience is possible.     

Vision influences on tactile discrimination of grating orientation

Studies have revealed a number of instances of cross-modal spatial interactions between vision and touch. However, few studies have addressed the possible role of cross-modal interactions in the identification of objects. This study assessed whether tactile pattern perception is affected by adding a concurrent visual stimulus. Participants were required to discriminate the orientation of tactile grating patterns, which were presented in synchrony or asynchrony with a task-irrelevant visual grating pattern that had the same or different orientation as the tactile pattern. The results demonstrated that accuracy in discriminating grating orientation was reduced when the tactile and visual gratings differed in orientation, but only when the two modalities were presented concurrently. The findings are discussed in terms of multisensory integration for processing of shape information.     

A physiological and behavioural study in cats of the effect of early visual experience with contours of a single orientation.

1. Three kittens were reared in visual environments that consisted of stripes at one of three orientations - horizontal, right oblique, or left oblique. Two additional cats were reared as controls. One of these matured viewing right and left oblique stripes on alternate days. The other experienced a normal visual environment. 2. Following the completion of rearing, and after several weeks of normal visual experience, behavioural testing of the stripe-reared animals demonstrated a deficit in visual acuity for orientations which were not present in the early visual environment. No comparable deficit emerged for either of the control cats. 3. Following 1-3 years of further, normal, visual experience, each of the cats was shipped separately to California where single units were recorded from area 17 of the visual cortex and an effort made to guess the early visual history of each animal which was unknown to the experimenters. Cell samples from each experimental cat and the normal control cat allowed the physiologist to guess their early visual experience correctly. The control cat which matured viewing orthogonal sets of oblique stripes on alternate days demonstrated a bias for horizontal contours in his cell sample. In contrast to units recorded from normal cats, about 80% of which are binocular, only about 30% of the cells recorded from the stripe-reared animals could be influenced by both eyes.     

Tactile co-activation improves detection of afferent spatial modulation

Tactile co-activation, i.e., synchronous stimulation of a region of skin, has been reported to improve tactile spatial acuity and expand the corresponding somatosensory cortical representation. The current study aimed to clarify the nature of the changes resulting from tactile co-activation, using three measures of tactile sensitivity obtained with controlled mechanical stimulation. One was the grating orientation (GR/OR) discrimination task, where acuity is indexed by the threshold groove width required for 75% correct discrimination between two orthogonal orientations of a grating on the fingerpad. Since this task may be susceptible to intensity cues due to tactile anisotropy, another acuity measure, the 3-dot task, was also used. In this task, the acuity threshold corresponds to 75% correct discrimination of the direction of offset of the central dot in a 3-dot array. In Experiment 1, co-activation failed to induce significant improvement in acuity with either of these measures. Experiment 2 employed both the GR/OR task, and a third measure based on discriminating a grooved from a smooth surface (SM/GV). While the former task demands detailed spatial resolution, the latter requires only that spatial modulation in the afferent population be detected. This experiment also included a control group. GR/OR performance did not significantly improve for either the control or experimental groups. There was, however, a significant improvement in SM/GV performance following co-activation for the experimental but not the control group. These findings indicate that the SM/GV task may be better suited than the GR/OR or 3-dot tasks for measuring changes in tactile sensitivity following co-activation.     

Orientation sensitivity of cat LGN neurones with and without inputs from visual cortical areas 17 and 18

Summary Orientation sensitivity was tested, using moving bars as stimuli, in 136 LGN cells in normal cats and 82 LGN cells in cats with areas 17 and 18 lesioned.The responses of most neurones showed some dependence on the orientation of the line stimulus. The orientation bias was more pronounced for long, narrow bars moving at rather slow velocities. Length-response curves revealed less end-inhibition along the optimum orientation than along the nonoptimum orientation. Thirty-two percent of the cells in the normal cats and 50% in the lesioned animals responded best to orientations within 10 ° of the vertical or horizontal. The oblique orientations were represented poorly in the lesioned group. Thus the corticogeniculate feedback may serve to confer a more uniform distribution of orientation preferences on the LGN.It is suggested that the orientation biases of LGN neurones may play a role in building orientation-selective cells in the visual cortex. Further, the preferences for horizontal and vertical orientations in the LGN may explain the preferences for these orientations reported for visual cortical cells.     

M.I.T./Canadian vestibular experiments on the Spacelab-1 mission: 2. Visual vestibular tilt interaction in weightlessness

Summary Adaptation to weightlessness includes the substitution of other sensory signals for the no longer appropriate graviceptor information concerning static spatial orientation. Visual-vestibular interaction producing roll circularvection was studied in weightlessness to assess the influence of otolith cues on spatial orientation. Preliminary results from four subjects tested on Spacelab-1 indicate that visual orientation effects were stronger in weightlessness than pre-flight. The rod and frame test of visual field dependence showed a weak post-flight increase in visual influence. Localized tactile cues applied to the feet in space reduced subjective vection strength.     

Influence of experience on orientation maps in cat visual cortex.

Experience is known to affect the development of ocular dominance maps in visual cortex, but it has remained controversial whether orientation preference maps are similarly affected by limiting visual experience to a single orientation early in life. Here we used optical imaging based on intrinsic signals to show that the visual cortex of kittens reared in a striped environment responded to all orientations, but devoted up to twice as much surface area to the experienced orientation as the orthogonal one. This effect is due to an instructive role of visual experience whereby some neurons shift their orientation preferences toward the experienced orientation. Thus, although cortical orientation maps are remarkably rigid in the sense that orientations that have never been seen by the animal occupy a large portion of the cortical territory, visual experience can nevertheless alter neuronal responses to oriented contours.     

Experience-dependent orientation plasticity in the visual cortex of rats chronically exposed to a single orientation

We used the intrinsic signal optical imaging technique to assess the effect of orientation-restricted visual experience on response properties of the rat visual cortex. We placed young animals wearing goggles fitted with plano-convex cylindrical lenses in a stimulus-enriched environment for 3 weeks. Experienced orientation was over-represented in the visual cortex, which was associated with the under-representation of orthogonal orientation. These findings suggest that chronic exposure to a single orientation can modify orientation preferences even in rats lacking in orderly arrangement of preferred orientations.     

A coordinate system for visual motion perception

The purpose of this research was to determine the reference axes used by the visual system to specify direction of motion of objects by the visual system at the perceptual level. Ten young adults aligned motion of a moving luminous dot on a computer display to body-fixed and external vertical and horizontal plane axes while operating in a dark room. Accuracy of aligning dot motion to earth-fixed vertical, a displayed luminous line (external visual axis) of varied orientations, and head and trunk longitudinal axes was tested in one experiment with the display in the vertical frontal plane. In a second experiment, dot motion was aligned to head and trunk anterior/posterior (A-P) axes and to an external visual axis presented on a horizontal computer screen. Head and trunk orientations were varied in the frontal plane (left/right tilt) when testing vertical plane axes and by rotation of the head and/or trunk about a vertical axis when testing horizontal plane axes. Perceptual errors were lowest when aligning to earth-fixed vertical in the vertical plane and to an external oblique line in the horizontal plane when head and trunk orientations were varied. Perceptions of horizontal plane motion direction were accurate relative to the trunk-fixed A-P axis when only head orientation was varied, but large errors were made when trunk orientation was varied. Proprioceptive influences on visual perceptions of motion direction were shown by a dependence of perceptual errors on trunk and neck orientations when aligning to all axes. Furthermore, when aligning motion to an external line, the errors depended on orientation of the line in addition to trunk and neck orientations, but not when aligning to intrinsic axes or earth-fixed vertical in the presence of an external line. We conclude that the visual motion system defines direction relative to earth-fixed vertical and an external horizontal reference axis when available. The trunk-fixed A-P axis can be used to accurately define motion direction when operating without an external reference if a neutral trunk orientation is maintained.     

Does the vestibular apparatus play a role in the development of the visual system?

1. The receptive field properties of visual cortical neurones were investigated in kittens that had been subjected to either unilateral or bilateral labyrinthectomy shortly after birth.

2. Two kittens were reared in a normal visual environment. Another two were reared in the dark with recurrent exposures to vertically oriented black and white stripes, which in normal kittens is known to bias the distribution of receptive field orientations.

3. For both normally reared and stripe-reared labyrinthectomized kittens, no differences were detected in cell types, preferred orientations, binocularity, columnar organization, or any other neuronal properties, compared with similarly reared intact kittens.

4. The failure to detect deficits in visual development after labyrinthectomy is discussed in relation to other reports of vestibular influences on the visual system of the adult cat.

     

Kinesthetic perception of visually specified axes

The purpose of this research was to determine whether human subjects could align the forearm more accurately to the orientation of an external object than to earth-fixed vertical and trunk-fixed anterior-posterior (a-p) axes. Ten young adults aligned the unseen forearm to earth-fixed vertical and trunk-fixed a-p axes, and to a visually presented rod (external visual axis) held by an experimenter in various oblique vertical and horizontal orientations. The head and trunk orientations were varied by left/right lateral flexion when aligning the forearm to vertical plane axes and by rotation about the vertical axis when aligning the forearm to horizontal plane axes. Perceptual errors for aligning the forearm to vertical plane axes were much lower when aligning the forearm to earth-fixed vertical than to an external visual axis positioned in a vertical plane. Furthermore, the perceptual errors for aligning the forearm to the visually presented rod were correlated with rod orientation while errors for aligning the forearm to vertical while viewing the rod were unaffected by rod orientations. Clearly, human subjects cannot use an oblique external visually presented axis to provide a frame of reference for accurate perception of forearm orientation in vertical planes. Perceptual errors were similar for aligning the forearm to the horizontal trunk-fixed a-p axis and external visual axis when head and trunk orientation were varied. These perceptual errors were not correlated with rod orientation in the horizontal plane, giving no evidence of bias toward the trunk or external visual axis in horizontal plane perception of forearm orientation. Thus, humans can use either the trunk-fixed a-p axis or the visually specified orientation of an external object as a frame of reference for the kinesthetic system to specify forearm orientation in the horizontal plane. Electronic Publication     

Influence of anisotropic compartments on magnetic field and electric potential distributions generated by artificial current dipoles inside a torso phantom

The purpose of this study is the analysis of the influence of anisotropic conductivity on magnetic fields and electric potentials by means of phantom measurements. An artificial rotating current dipole was placed in the middle of an anisotropic skein arrangement in a torso phantom filled with saline solution. The signal strength and the change of the shape of potential and field patterns due to anisotropic volume conduction were investigated. Different directions of the dipole were compared to corresponding orientations of measured fields and potentials (angle difference). For electric and magnetic data, the angle difference between observed signal orientations and true dipole orientations continuously increased with the angle between dipole and anisotropy (up to 80 degrees ) and then decreased back to zero at their orthogonal orientation. Both signal strengths decreased about 10% with an increasing angle between dipole and anisotropy. While the magnetic field showed a generally stronger shape change, the changed shape of the electric potential showed similarity to an extended source. Our phantom study demonstrated that anisotropic compartments influence directions, amplitudes and shapes of potentials and fields at different degrees. We concluded that anisotropic structures should be considered in volume conductor modelling, when source orientation, extension and strength are of interest.     

The effect of seed orientation deviations on the quality of 125I prostate implants.

We quantified the effect of seed orientation deviations on five prostate seed implant cases at our institution. While keeping their positions fixed, the iodine-125 seeds were assigned orientations sampled from a realistic probability distribution derived from the post-implant radiographs of ten patients. Dose distributions were calculated with both a model that explicitly includes anisotropy (TG43 anisotropy function) and a point source model (TG43 anisotropy factor). Orientation deviations had only a small influence on prostate dose-volume histograms: the 95% confidence intervals on the volumes receiving 100%, 150% and 200% dose were at most +/-0.8%, +/-1.1% and +/-0.6% of the prostate volume, respectively. The dose-volume histograms of anisotropic seed distributions were marginally better than those with isotropic point-source seeds. Anisotropy caused a displacement of cold spots (regions receiving <100% of the prescribed dose) in <1% of the prostate volume. Our results indicate no net benefit to prostate dosimetry in using more isotropic seeds. Furthermore, we propose a new 'weighted anisotropy function' to better account for the effects of anisotropy when seed orientation is unknown. Conceptually, the TG43 anisotropy factor described in AAPM TG43 averages the effect of anisotropy over all solid angles, with the implicit assumption that all seed orientations are equally probable. In prostate implants, however, seeds are preferentially oriented parallel to the needle axis. The proposed weighted anisotropy function incorporates this non-uniform probability.     

Double tactile sensations evoked by a single visual stimulus on a rubber hand

The classical rubber hand illusion involves individuals misattributing tactile sensations ‘felt’ by their real hand hidden from view to a rubber prosthetic hand that they ‘see’ being tactilely stimulated in synchrony. However, from previous studies, it is not clear whether individuals actually feel the tactile sensation in the rubber hand, real hand, or both because the same part of the rubber and real hands were stimulated simultaneously. Here, we attempted to isolate subjects’ sensations attributed to the rubber hand from those sensed in the real hand by placing the rubber and real hands in opposing orientations (e.g., palm up versus dorsum up). Interestingly, half of the subjects reported two tactile sensations for one visual stimulus, that is, one in the rubber finger stimulated visually with a light source and one in the real finger overlapping the rubber finger. This finding suggests that the tactile sensation induced by the visual stimulus is referred to the rubber hand and real hand simultaneously. Thus, both visuo-spatial and somatic codes are used in the localization of tactile sensation in the rubber hand illusion.