Enhancement of three-dimensional perception of numerical hologram reconstructions of real-world objects by motion and stereo.

We investigated the question of how the perception of three-dimensional information reconstructed numerically from digital holograms of real-world objects, and presented on conventional displays, depends on motion and stereoscopic presentation. Perceived depth in an adjustable random pattern stereogram was matched to the depth in hologram reconstructions. The objects in holograms were a microscopic biological cell and a macroscopic metal coil. For control, we used real physical objects in additional to hologram reconstructions of real objects. Stereoscopic presentation increased perceived depth substantially in comparison to non-stereoscopic presentation. When stereoscopic cues were weak or absent e.g. because of blur, motion increased perceived depth considerably. However, when stereoscopic cues were strong, the effect of motion was small. In conclusion, for the maximization of perceived three-dimensional information of holograms on conventional displays, it seems highly beneficial to use the combination of motion and stereoscopic presentation.

Synthesis and display of dynamic holographic 3D scenes with real-world objects.

A 3D scene is synthesized combining multiple optically recorded digital holograms of different objects. The novel idea consists of compositing moving 3D objects in a dynamic 3D scene using a process that is analogous to stop-motion video. However in this case the movie has the exciting attribute that it can be displayed and observed in 3D. We show that 3D dynamic scenes can be projected as an alternative to complicated and heavy computations needed to generate realistic-looking computer generated holograms. The key tool for creating the dynamic action is based on a new concept that consists of a spatial, adaptive transformation of digital holograms of real-world objects allowing full control in the manipulation of the object's position and size in a 3D volume with very high depth-of-focus. A pilot experiment to evaluate how viewers perceive depth in a conventional single-view display of the dynamic 3D scene has been performed.

Colour vision defects in occupational chronic solvent encephalopathy.

Occupational chronic solvent encephalopathy (CSE) is associated with a number of neurobehavioural disorders including defects of visual perception. The purpose of this study was to characterize colour vision defects in CSE patients. Colour vision was tested in bright illumination with the Farnsworth-Munsell 100-hue test in workers who had CSE due to occupational exposure to common industrial solvents. Before assessing colour vision, the subjects' ocular health and visual functions were evaluated. On the basis of this evaluation, 36 subjects with healthy eyes were selected and their colour vision was tested monocularly. The colour vision performance of the patient group was, statistically, significantly inferior to that of a control group matched by age at a group level. A mixed form of reduced colour sensitivity was found in 42% (n=15/36) of the cases, affecting the entire range of Munsell hues. No association was found between the length and intensity of exposure and colour vision performance. Our results show that CSE patients can have significantly impaired colour discrimination ability, although their eyes are healthy and their other visual functions are normal. This may indicate toxic damage to higher level visual processing, possibly the colour selective regions of the cerebral cortex.

Visual search and eye movements in patients with chronic solvent-induced toxic encephalopathy.

Various aspects of visual perception have been found to be impaired in patients with occupational chronic solvent-induced toxic encephalopathy (CSE). The purpose of the study was to characterise the changes in eye movements and visual search performance in CSE patients. We measured eye movements of 13 CSE patients and 22 healthy controls during dynamic visual search task by using a fast video eye tracker. The task was to search for and identify a target letter among numerals presented in a rectangular stimulus matrix (3x3-10x10 items). Threshold search time, i.e. the duration of stimulus presentation required for identifying the target with a given probability was determined by using a psychophysical staircase method. The visual search times of the CSE patients were clearly longer, and they needed considerably more eye fixations than healthy controls to find the target. Thus, their reduced performance in this task was mainly related to the reduction in the number of items which could be processed during a single eye fixation (perceptual span). This reduction probably reflects a limited capacity of visual attention, since visual acuity, contrast sensitivity, and the oculomotor saccade velocity were found to be normal. The results suggest that motor slowness or low-level visual factors do not explain the poor performance of CSE patients in visual search tasks. The results are also discussed with respect to the effects of education, and compared to the performance in the widely used neuropsychological Trail Making Test, which uses similar stimuli and requires visual search.

Effects of long-term occupational solvent exposure on contrast sensitivity and performance in visual search.

Sensitive and easily applicable screening tests are greatly needed for the early detection of nervous system dysfunction in people working with neurotoxic substances. Previous studies have shown that long-term solvent exposure may cause deficits in visual perception. We, therefore, studied the effects of long-term occupational solvent exposure and chronic encephalopathy on performance in three vision tests novel in the present context. Two visual search tasks were used: the letter search test measured the speed of finding a letter in an array of numerals, and the pop-out search test required the observer to detect the presence or absence of a tilted line segment in an array of vertical line segments. With the letter contrast sensitivity test we measured the contrast threshold for the identification of band-pass filtered letters. Before testing, comprehensive eye examination was carried out to reveal any structural or functional abnormality and to ensure correct refraction. The patients had healthy eyes, 2 out of 14 had reduced contrast sensitivity (Vistech) and 5 out of 14 had deficits in colour vision (FM 100). In both visual search tasks, the patients were statistically highly significantly (p<0.001) slower than the age-matched control observers. Instead, in the contrast sensitivity test, the difference between the patient and the control group was small relative to normal variability although still statistically significant (p<0.05). The results suggest that visual search tests can be useful in evaluating and characterising the effects of long-term solvent exposure on visual perception. Because our patients' letter contrast sensitivity was only moderately deteriorated, it seems that the observed defect of visual search cannot be explained by deteriorated letter identification alone, although it can be a contributory factor. Rather, the finding suggests that the speed by which visual information is transmitted and/or processed in the central visual system has become considerably slower.

Utilisation of spatial frequency information in face search.

In previous studies the utilisation of spatial frequency information in face perception has been investigated by using static recognition tasks. In this study we used a visual search task, which requires eye movements and fast identification of previously learned facial photographs. Using Fourier phase randomisation, spatial information was selectively removed without changing the amplitude spectrum of the image. Fourier phase was randomised within one-octave wide bands of nine different centre spatial frequencies (2-32 c/face width, 0.63-10.1 c/deg). In a control condition no randomisation was used. All stimuli had similar contrast. Search times and eye movements during the search were measured. The removal of spatial information by phase randomisation at medium spatial frequencies resulted in a considerable increase of search times. In the main experiment the maximum of the search times occurred between 8 and 11 c/face width. The number of eye fixations behaved similarly. In an additional experiment with a threefold viewing distance the search times increased and the maximum of the search times shifted slightly to lower object spatial frequencies (5.6-8 c/face width). This suggests that the band of spatial frequencies used in face search is not completely scale invariant. The results show that information most important to face search is located at a limited band of mid spatial frequencies. This is consistent with earlier studies, in which non-dynamical face recognition tasks and low-contrast stimuli have been used.

Eye movements in the visual search of word lists.

The word identification span refers to the area of the visual field in which words can be identified during a single fixation. The purpose of the study was to estimate the vertical word identification span in a visual word search task, in which words were arranged in a vertical list. In addition, we studied the effect of list layout (orientation, length, and line spacing) on the speed of search and eye movements. The task of the observer was to identify a target word in a word list, where the other words were distracters. Threshold search time, that is, stimulus presentation time for correct identification at a probability level of 0.79, was determined by using a multiple alternative staircase method. Eye movements were recorded simultaneously. The results showed that, in vertical lists, 4-5 words could be identified during a single fixation. Thus, the vertical word identification span was 4-5 character spaces, whereas according to previous studies the horizontal word identification span is about 10 character spaces, which corresponds to 1-2 words. There were fewer fixations and the saccade amplitudes were smaller for vertical than for horizontal lists of the same length. However, search times did not depend on list orientation. This was due to longer fixation times for vertical lists. Further, since average fixation duration for vertical lists was longer than for horizontal lists, processing time seems to depend on the number of items within the span.

Face recognition and cortical responses: effect of stimulus duration.

To clarify the relationship between face perception and cortical activation, we manipulated the face recognition performance of 9 subjects by varying the duration (DUR) of stimuli while cortical neuromagnetic responses were recorded. A face image replaced a continuous pixel-noise mask for 17-200 ms, and the subject reported which of the pre-learned faces had been presented. Two cortical responses were clearly stronger to intact than phase-scrambled faces: the temporo-occipital response peaking at 140-200 ms (M170) and a more widely distributed response peaking at 200-500 ms (M300). For the shortest DURs (17-33 ms), face recognition was at chance level and the cortical responses negligible. For DURs of 50-83 ms, the proportion of recognized faces as well as the strength of cortical responses increased steeply. Recognition performance saturated at DURs of around 100 ms, whereas cortical responses continued to increase until the longest DUR of 200 ms. Amplitudes of both M170 and M300 were thus tightly correlated with recognition performance (r=0.98), but comparison of the increment rates as a function of DUR showed the recognition performance to have an even closer similarity to M170 than to M300. In single-trial analysis the variability of response strengths increased in a direct proportion to response amplitude, demonstrating the averaged responses to be composed of graded rather than of all-or-nothing-type single responses.

Estimation of temporal resolution of object identification in human vision.

The purpose of the study was to estimate the temporal processing capacity of human object identification under different stimulus conditions. Objects, either facial images or characters, were shown in a rapid sequence on a computer display using a rapid serial visual presentation (RSVP) method. One of the images was a target and the other images were distracters. The task of the observer was to identify the target. A staircase algorithm was used to determine the threshold frequency of image presentation in the RSVP sequence. The threshold frequency was determined as a function of image contrast, size, and mean luminance. The results showed that the threshold frequency, around 10 Hz for faces (100 ms per face) and about 25 Hz for characters (40 ms per character), was independent of contrast and size at medium and high contrast values, medium and large sizes, and high luminances, but decreased at very low contrasts or small sizes and medium or low levels of luminance. Computer simulations with a model, in which temporal integration limited perceptual speed, suggest that the experimentally found difference in processing time for faces and characters is not due to the physical differences of these stimulus types, but it seems that face-specific sites in the brain process facial information slower than object-specific areas process character information. Contrast, size, and luminance affect the signal-to-noise ratio and the temporal characteristics of low-level neural signal representation. Thus, the results suggest that at low contrasts, low luminances and small sizes, the processing speed of object identification is limited by low-level factors, while at high contrasts and luminances, and at large sizes, processing speed is limited by high-order processing stages. Processing speed seems to depend on stimulus type so that for faces processing is slower than for characters.

Face recognition and cortical responses show similar sensitivity to noise spatial frequency.

To find cortical correlates of face recognition, we manipulated the recognizability of face images in a parametric manner by masking them with narrow-band spatial noise. Face recognition performance was best at the lowest and highest noise spatial frequencies (NSFs, 2 and 45 c/image, respectively), and degraded gradually towards central NSFs (11-16 c/image). The strength of the 130-180 ms neuromagnetic response (M170) in the temporo-occipital cortex paralleled the recognition performance, whereas the mid-occipital response at 70-120 ms acted in the opposite manner, being strongest for the central NSFs. To noise stimuli without faces, M170 was small and rather insensitive to NSF, whereas the mid-occipital responses resembled closely the responses to the combined face and noise stimuli. These results suggest that the 100 ms mid-occipital response is sensitive to the central spatial frequencies that are critical for face recognition, whereas the M170 response is sensitive to the visibility of a face and closely related to face recognition.

How many faces can be processed during a single eye fixation?

The purpose of our study was to estimate the perceptual span for facial information: how many faces can be processed during a single eye fixation. We used a visual-search task, in which the targets and distractors were facial photographs. The task of the observer was to search for and identify a target face in an array of faces. We measured the time needed for one search--threshold search time--by using a multiple-alternative staircase method. The threshold represents the duration of stimulus presentation at which the probability of correct responses was 79%. The array size was varied from 2 x 2 to 8 x 8 faces. Simultaneously with the performance measurements we measured eye movements with a video eye tracker. We found that threshold search time increased with increasing set size nearly linearly. The number of fixations also increased linearly from unity at the smallest set size to about fifteen at the largest set size. The result of 2 x 2 faces during a single fixation gave an estimate of 4 faces for the perceptual span. If, on average, only half of the elements had to be scanned for finding the target, 15 fixations at the largest set size (8 x 8) gave another estimate of 2.13 faces. The mean fixation duration was around 200 ms. Thus, the results suggest that 2-4 faces can be processed during one fixation of about 200 ms.