Primate retina trades single-photon detection for high-fidelity contrast encoding.

How the spike output of the retina enables human visual perception is not fully understood. Here, we address this at the sensitivity limit of vision by correlating human visual perception with the spike outputs of primate ON and OFF parasol (magnocellular) retinal ganglion cells in tightly matching stimulus conditions. We show that human vision at its ultimate sensitivity limit depends on the spike output of the ON but not the OFF retinal pathway. Consequently, nonlinear signal processing in the retinal ON pathway precludes perceptual detection of single photons in darkness but enables quantal-resolution discrimination of differences in light intensity.

Sound-action symbolism in relation to precision manipulation and whole-hand grasp usage.

It has been suggested that actions can provide a fruitful conceptual context for sound symbolism phenomena, and that tight interaction between manual and articulatory processes might cause that hand actions, in particular, are sound-symbolically associated with specific speech sounds. Experiment 1 investigated whether novel words, built from speech sounds that have been previously linked to precision or power grasp responses, are implicitly associated with perceived actions that present precision manipulation or whole-hand grasp tool-use or the corresponding utilisation pantomimes. In the two-alternative forced-choice task, the participants were more likely to match novel words to tool-use actions and corresponding pantomimes that were sound-symbolically congruent with the words. Experiment 2 showed that the same or even larger sound-action symbolism effect can be observed when the pantomimes present unfamiliar utilisation actions. Based on this we propose that the sound-action symbolism might originate from the same sensorimotor mechanisms that process the meaning of iconic gestural signs. The study presents a novel sound-action phenomenon and supports the view that hand-mouth interaction might manifest itself by associating specific speech sounds with grasp-related utilisations.

Front Is High and Back Is Low: Sound-Space Iconicity in Finnish.

Previous investigations have shown various interactions between spatial concepts and speech sounds. For instance, the front-high vowel [i] is associated with the concept of forward, and the back-high vowel [o] is associated with the concept of backward. Three experiments investigated whether the concepts of forward/front and backward/back are associated with high- and low-pitched vocalizations, respectively, in Finnish. In Experiments 1 and 2, the participants associated the high-pitched vocalization with the forward-directed movement and the low-pitched vocalizations with the backward-directed movement. In Experiment 3, the same effect was observed in relation to the concepts of front of and back of. We propose that these observations present a novel sound-space symbolism phenomenon in which spatial concepts of forward/front and backward/back are iconically associated with high- and low-pitched speech sounds. This observation is discussed in relation to the grounding of semantic knowledge of these spatial concepts in the movements of articulators such as relative front/back-directed movements of the tongue.

The retinal and perceived locus of fixation in the human visual system.

Due to the dramatic difference in spatial resolution between the central fovea and the surrounding retinal regions, accurate fixation on important objects is critical for humans. It is known that the preferred retinal location (PRL) for fixation of healthy human observers rarely coincides with the retinal location with the highest cone density. It is not currently known, however, whether the PRL is consistent within an observer or is subject to fluctuations and, moreover, whether observers' subjective fixation location coincides with the PRL. We studied whether the PRL changes between days. We used an adaptive optics scanning laser ophthalmoscope to project a Maltese cross fixation target on an observer's retina and continuously imaged the exact retinal location of the target. We found that observers consistently use the same PRL across days, regardless of how much the PRL is displaced from the cone density peak location. We then showed observers small stimuli near the visual field location on which they fixated, and the observers judged whether or not the stimuli appeared in fixation. Observers' precision in this task approached that of fixation itself. Observers based their judgment on both the visual scene coordinates and the retinal location of the stimuli. We conclude that the PRL in a normally functioning visual system is fixed, and observers use it as a reference point in judging stimulus locations.

Perceived emotional expressions of composite faces.

The eye and mouth regions serve as the primary sources of facial information regarding an individual's emotional state. The aim of this study was to provide a comprehensive assessment of the relative importance of those two information sources in the identification of different emotions. The stimuli were composite facial images, in which different expressions (Neutral, Anger, Disgust, Fear, Happiness, Contempt, and Surprise) were presented in the eyes and the mouth. Participants (21 women, 11 men, mean age 25 years) rated the expressions of 7 congruent and 42 incongruent composite faces by clicking on a point within the valence-arousal emotion space. Eye movements were also monitored. With most incongruent composite images, the perceived emotion corresponded to the expression of either the eye region or the mouth region or an average of those. The happy expression was different. Happy eyes often shifted the perceived emotion towards a slightly negative point in the valence-arousal space, not towards the location associated with a congruent happy expression. The eye-tracking data revealed significant effects of congruency, expressions and interaction on total dwell time. Our data indicate that whether a face that combines features from two emotional expressions leads to a percept based on only one of the expressions (categorical perception) or integration of the two expressions (dimensional perception), or something altogether different, strongly depends upon the expressions involved.

Luminance gradient at object borders communicates object location to the human oculomotor system.

The locations of objects in our environment constitute arguably the most important piece of information our visual system must convey to facilitate successful visually guided behaviour. However, the relevant objects are usually not point-like and do not have one unique location attribute. Relatively little is known about how the visual system represents the location of such large objects as visual processing is, both on neural and perceptual level, highly edge dominated. In this study, human observers made saccades to the centres of luminance defined squares (width 4 deg), which appeared at random locations (8 deg eccentricity). The phase structure of the square was manipulated such that the points of maximum luminance gradient at the square's edges shifted from trial to trial. The average saccade endpoints of all subjects followed those shifts in remarkable quantitative agreement. Further experiments showed that the shifts were caused by the edge manipulations, not by changes in luminance structure near the centre of the square or outside the square. We conclude that the human visual system programs saccades to large luminance defined square objects based on edge locations derived from the points of maximum luminance gradients at the square's edges.

Efficient Avoidance of the Penalty Zone in Human Eye Movements.

People use eye movements extremely effectively to find objects of interest in a cluttered visual scene. Distracting, task-irrelevant attention capturing regions in the visual field should be avoided as they jeopardize the efficiency of search. In the current study, we used eye tracking to determine whether people are able to avoid making saccades to a predetermined visual area associated with a financial penalty, while making fast and accurate saccades towards stimuli placed near the penalty area. We found that in comparison to the same task without a penalty area, the introduction of a penalty area immediately affected eye movement behaviour: the proportion of saccades to the penalty area was immediately reduced. Also, saccadic latencies increased, but quite modestly, and mainly for saccades towards stimuli near the penalty area. We conclude that eye movement behaviour is under efficient cognitive control and thus quite flexible: it can immediately be adapted to changing environmental conditions to improve reward outcome.

The eyes like their targets on a stable background.

In normal human visual behavior, our visual system is continuously exposed to abrupt changes in the local contrast and mean luminance in various parts of the visual field, as caused by actual changes in the environment, as well as by movements of our body, head, and eyes. Previous research has shown that both threshold and suprathreshold contrast percepts are attenuated by a co-occurring change in the mean luminance at the location of the target stimulus. In the current study, we tested the hypothesis that contrast targets presented with a co-occurring change in local mean luminance receive fewer fixations than targets presented in a region with a steady mean luminance. To that end we performed an eye-tracking experiment involving eight observers. On each trial, after a 4 s adaptation period, an observer's task was to make a saccade to one of two target gratings, presented simultaneously at 7° eccentricity, separated by 30° in polar angle. When both targets were presented with a steady mean luminance, saccades landed mostly in the area between the two targets, signifying the classic global effect. However, when one of the targets was presented with a change in luminance, the saccade distribution was biased towards the target with the steady luminance. The results show that the attenuation of contrast signals by co-occurring, ecologically typical changes in mean luminance affects fixation selection and is therefore likely to affect eye movements in natural visual behavior.

Fovea-periphery axis symmetry of surround modulation in the human visual system.

A visual stimulus activates different sized cortical area depending on eccentricity of the stimulus. Here, our aim is to understand whether the visual field size of a stimulus or cortical size of the corresponding representation determines how strongly it interacts with other stimuli. We measured surround modulation of blood-oxygenation-level-dependent signal and perceived contrast with surrounds that extended either towards the periphery or the fovea from a center stimulus, centered at 6° eccentricity. This design compares the effects of two surrounds which are identical in visual field size, but differ in the sizes of their cortical representations. The surrounds produced equally strong suppression, which suggests that visual field size of the surround determines suppression strength. A modeled population of neuronal responses, in which all the parameters were experimentally fixed, captured the pattern of results both in psychophysics and functional magnetic resonance imaging. Although the fovea-periphery anisotropy affects nearly all aspects of spatial vision, our results suggest that in surround modulation the visual system compensates for it.

The effect of mean luminance change and grating pedestals on contrast perception: model simulations suggest a common, retinal, origin.

The percept of a contrast target is substantially affected by co-occurring changes in mean luminance or underlying ("pedestal") contrast elements. These two types of modulatory effects have traditionally been studied as separate phenomena. However, regardless of different higher-level mechanisms, both classes of phenomena will necessarily also depend on shared mechanisms in the first stages of vision, starting with the primary responses of photoreceptors. Here we present model simulations showing that important aspects of both classes may be explained by the temporal dynamics of photoreceptor responses read by integrate-and-fire operators. The model is physiologically justified and all its parameters are constrained by experimental evidence. Although there remains plenty of room for additional mechanisms to shape the exact quantitative realization of the perceptual functions in different situations, we suggest that signature features may be inherited from primary retinal signaling.

Effects of mean luminance changes on human contrast perception: contrast dependence, time-course and spatial specificity.

BACKGROUND: When we are viewing natural scenes, every saccade abruptly changes both the mean luminance and the contrast structure falling on any given retinal location. Thus it would be useful if the two were independently encoded by the visual system, even when they change simultaneously. Recordings from single neurons in the cat visual system have suggested that contrast information may be quite independently represented in neural responses to simultaneous changes in contrast and luminance. Here we test to what extent this is true in human perception. METHODOLOGY/PRINCIPAL FINDINGS: Small contrast stimuli were presented together with a 7-fold upward or downward step of mean luminance (between 185 and 1295 Td, corresponding to 14 and 98 cd/m(2)), either simultaneously or with various delays (50-800 ms). The perceived contrast of the target under the different conditions was measured with an adaptive staircase method. Over the contrast range 0.1-0.45, mainly subtractive attenuation was found. Perceived contrast decreased by 0.052±0.021 (N = 3) when target onset was simultaneous with the luminance increase. The attenuation subsided within 400 ms, and even faster after luminance decreases, where the effect was also smaller. The main results were robust against differences in target types and the size of the field over which luminance changed. CONCLUSIONS/SIGNIFICANCE: Perceived contrast is attenuated mainly by a subtractive term when coincident with a luminance change. The effect is of ecologically relevant magnitude and duration; in other words, strict contrast constancy must often fail during normal human visual behaviour. Still, the relative robustness of the contrast signal is remarkable in view of the limited dynamic response range of retinal cones. We propose a conceptual model for how early retinal signalling may allow this.

Area summation in human visual system: psychophysics, fMRI, and modeling.

Contextual modulation is a fundamental feature of sensory processing, both on perceptual and on single-neuron level. When the diameter of a visual stimulus is increased, the firing rate of a cell typically first increases (summation field) and then decreases (surround field). Such an area summation function draws a comprehensive profile of the receptive field structure of a neuron, including areas outside the classical receptive field. We investigated area summation in human vision with psychophysics and functional magnetic resonance imaging (fMRI). The stimuli were drifting sine wave gratings similar to those used in previous macaque single-cell area summation studies [corrected]. A model was developed to facilitate comparison of area summation in fMRI to area summation in psychophysics and single cells. The model consisted of units with an antagonistic receptive field structure found in single cells in the primary visual cortex. The receptive field centers of the model neurons were distributed in the region of the visual field covered by a single voxel. The measured area summation functions were qualitatively similar to earlier single-cell data. The model with parameters derived from psychophysics captured the spatial structure of the summation field in the primary visual cortex as measured with fMRI. The model also generalized to a novel situation in which the neural population was displaced from the stimulus center. The current study shows that contextual modulation arises from similar spatially antagonistic and overlapping excitatory and inhibitory mechanisms, both in single cells and in human vision.

Time course of suppression by surround gratings: highly contrast-dependent, but consistently fast.

Timing is critical for the effectiveness of a modulating surround signal. In this study, the optimal timing of a suppressing surround signal was measured psychophysically in human subjects. The perceived contrast of a fixated 1-deg circular patch of vertical sinusoidal grating (the target: 4 cpd, Michelson contrast 0.2) was measured as a function of the onset asynchrony between the target and an annular "surround" grating with the same orientation and spatial frequency. The contrast and area of the surround stimulus were varied parametrically. The suppressive signal peaked at earlier times the higher the surround contrast (0.1-0.4), following a function consistent with the contrast-dependence of retinal response dynamics. Increasing the area of the surround grating also moved peak suppression to earlier times. At ca. 2 deg annulus outer diameter the time to peak of the suppressive signal was shortest, although its amplitude grew with annulus area even beyond that. When both the contrast and the area of the centre and surround gratings were equal, suppression was maximal if the surround stimulus was presented ca. 5 ms before the target. Such a short delay of suppression is consistent with a neural implementation based on feedforward-feedback connections, but not with horizontal connections.

Evidence for the different additivity of the temporal and frontal generators of mismatch negativity: a human auditory event-related potential study.

The auditory sensory-memory mechanisms in the human brain were investigated using the mismatch negativity (MMN) component of the event-related potential. MMNs were recorded to stimuli deviating from the repetitive standard stimuli simultaneously either in one or two features (frequency, intensity). If the processing of these two features is independent of each other, the MMN to the double deviant should equal the sum of the MMNs elicited by the corresponding single deviants. The double-deviant MMNs were found to be additive at the electrode sites below the Sylvian fissure but not at the frontal scalp areas. The results suggest that the temporal subcomponent of MMN is additive whereas the frontal is non-additive. The pattern of results was similar in ignore and attend conditions, suggesting that the components were not attentionally modulated.