The use of visual feedback is independent of visual awareness: evidence from visual extinction

Milner and Goodale (The visual brain in action, Oxford Press, 1995) made a distinction between vision for perception and vision for action. In contrast to perception, many action tasks have strict temporal constraints, which can only be met if the visual information is relayed directly to the motor system without first passing through a conscious decision making process. Milner and Goodale therefore predict that visual stimuli do not have to reach visual awareness in order to guide rapid motor responses. Online visual feedback provides a good example of visual information that is used under tight temporal constraints to guide rapid motor responses. Online visual feedback provides information about the position of the moving limb. This information can be used to improve the accuracy of our movements. If vision for action operates independently of visual awareness, visual feedback should be beneficial even if the subject is unaware of this information. We tested this prediction in a patient (V.E.) with left-sided visual extinction, a condition in which a visual stimulus typically fails to reach awareness if a second stimulus is presented simultaneously at a more rightward location. V.E. was asked to point towards a central target with his left hand. In some trials a light-emitting diode (LED) provided brief visual feedback from the moving hand. However, in the majority of trials, V.E. was unaware of this LED, due to his extinction. His performance was nevertheless significantly better when visual feedback was present, regardless of whether or not the information was available for verbal report. We conclude that visual awareness is not essential for the effective use of online visual feedback. The study was carried out at: Cognitive Neuroscience Research Unit (CNRU), Wolfson Research Institute, University of Durham, UK     

A signal detection study of the Colavita visual dominance effect

When presented with auditory, visual, or bimodal audiovisual stimuli in a speeded detection/discrimination task, participants fail to respond to the auditory component of the bimodal targets significantly more often than they fail to respond to the visual component. Signal detection theory (SDT) was used to explore the contributions of perceptual (sensitivity shifts) and decisional (shifts in response criteria) factors to this effect, known as the Colavita visual dominance effect. Participants performed a version of the Colavita task that had been modified to allow for SDT analyses. The participants had to detect auditory and visual targets (presented unimodally or bimodally) at their individually determined 75% detection thresholds. The results showed a significant decrease in participants’ sensitivity to auditory stimuli when presented concurrently with visual stimuli (in the absence of any significant change in their response criterion), suggesting that Colavita visual dominance does not simply reflect a decisional effect, but can be explained, at least in part, as a truly perceptual phenomenon. The decrease in sensitivity (to auditory stimuli) may be attributable to the exogenous capture of participants’ attention by the visual component of the bimodal target, thus leaving fewer attentional resources for the processing of the auditory stimulus. The reduction in auditory sensitivity reported here may be considered an example of crossmodal masking.     

Modulation of excitatory and inhibitory circuits for visual awareness in the human right parietal cortex

The balance of specific patterns of excitation and inhibition in critical regions of both hemispheres could be relevant in orienting attention over the extrapersonal space. In the present study a group of normal subjects had to detect small rectangular stimuli presented briefly on a computer screen in three different conditions: unilateral presentation either to left or right visual periphery or bilateral simultaneous presentation. Paired transcranial magnetic stimulation (TMS), was applied over the right parietal cortex 150 ms after the presentation of the visual stimuli with different inter-stimulus intervals (ISIs: 1, 3,5 and 10 ms). When paired TMS was applied 150 ms, but not 100 ms, after simultaneous visual presentation, the number of failures in detecting left targets increased compared to the single-pulse condition if the ISI was 3 ms; on the contrary, it decreased if the ISI was 5 ms. No effects were seen when paired pulses of the same intensity were delivered. These findings provide evidence of a supramodal-specific pattern of excitability of the right posterior parietal cortex in processing visuospatial information.     

An ERP study of change detection,change blindness,and visual awareness

Electrophysiological correlates of change detection and change blindness were studied in 12 observers. The ERP difference between detected changes and undetected changes was considered an electrophysiological correlate of visual awareness. Two distinct electrophysiological responses correlated with the awareness of change. First, awareness was associated with a negative amplitude shift at posterior sites around 200 ms after the change in the stimulus. The latency of the negative shift varied as a function of the task difficulty and the speed of becoming aware of the change. Second, ERPs to detected changes became more positive as compared with undetected changes around 400 ms after the change in the stimulus, peaking at parietal sites. We suggest that the earlier negativity is associated with a change in the content of visual awareness, whereas the later positivity may reflect more global processes needed in decision making and action planning.     

基于神经网络的VEP波形自动识别方法

目的：研究基于神经网络的视觉诱发电位波形的自动识别方法,实现Ｐ１００波的自动识别。方法：从历年来临床闪光ＶＥＰ检查记录中选择出经专家用视觉方法判断有Ｐ１００波存在的记录文件,记录专家判别的Ｐ１００波和比较波的数据,以此记录材料作为神经网络的学习样本,建立采用反向回归算法的神经网络。选取波形特征参数作为神经网络的输入参数,用具有不同隐层结点数的神经网络对相同样本进行学习和检验。结果：神经网络对学习样本的判别一致率为：Ａ组９８８％,Ｂ组９８８％;对检验样本的判别一致率为：Ａ组平均９２２％,Ｂ组平均９６０％。以全体材料为学习样本时网络的判别一致率为９７６％～９８２％,其中不同隐结点数的网络判别一致率差异极显著（χ２检验,χ２＝９．６３＞６．６３,Ｐ＜０．０１）。结论：专家的判断原则可以为神经网络所阐明     

Timing of the earliest ERP correlate of visual awareness

The earliest reliably occurring event-related brain potential (ERP) correlate of visual awareness (visual awareness negativity, VAN) emerges after 100 ms and peaks between 200 and 300 ms from stimulus onset. In a study using low-contrast stimuli, VAN was significantly delayed, peaking at 460 ms (V. Ojanen, A. Revonsuo, & M. Sams, 2003). In that study physical differences between the conscious and nonconscious stimuli may have confounded the results. Here we explored whether VAN is similarly delayed for physically identical stimuli. We presented low-contrast stimuli near an individually determined subjective contrast threshold. A delayed VAN peaked at 400 ms at occipito-temporal sites to subjectively perceived stimuli. Our results support the interpretation that VAN is the earliest ERP correlate of phenomenal visual awareness. The electrophysiological processes eliciting VAN may become delayed as a function of the difficulty of the early perceptual discrimination.     

Local (focussed) and global (distributed) visual processing in hemispatial neglect

In the present study we set out to investigate deficits of focussed and distributed attention (and their interaction) in brain-damaged patients. To this purpose, four left brain damaged (LBD) patients without signs of hemispatial neglect and six right brain damaged (RBD) patients with variable signs of hemispatial neglect were tested by means of an experimental paradigm comprising two embedded tasks performed on the same visual array. The first task (i.e. counting the number of shapes, 1–4, briefly displayed) mainly involved distributed attention rather than focussed attention. The second task was a typical target detection task, which emphasized the detailed analysis of each element in the array, thus mainly tapping focussed attention. Results clearly showed that: (1) LBD patients are slightly impaired at directing focussed attention to the contralesional visual hemifield; (2) in comparison to LBD patients, RBD patients with mild neglect show an exaggerated difficulty in orienting focussed attention toward the contralesional side, while they are similarly unimpaired in the deployment of distributed attention, i.e. in global processing; (3) RBD patients with the most severe neglect suffer from a deficit of both local and global visual processing, i.e. of both focussed and distributed attention. Taken together, these observations indicate that focussed and distributed components of visual attentional processing may be differentially affected in left and right brain-damaged patients with and without neglect.     

Boosting up gamma-band oscillations leaves target-stimulus in masking out of awareness: Explaining an apparent paradox

Boost up gamma-band neuronal oscillations have been interpreted as a correlate of pertinent stimulus awareness. The validity of this observation-based conclusion can be rigorously tested if the basic methodological rule of investigating neural correlates of consciousness (NCC) is followed: the varying contents of consciousness should be contrasted with invariant stimulation. We asked whether reliable gamma-band oscillations recorded from primary visual cortex appear as signatures of target awareness in metacontrast masking with invariant stimulation parameters. Surprisingly, clear target-stimuli awareness was associated with less expressed gamma power. However, because when target awareness was effectively masked more gamma power of the EEG response was found, and because this gamma-boost emerged at the post-target time when mask information was presented, we were able to explain our results as target substitution in consciousness by mask representation due to enhanced mask processing.     

Decline in visual attention and spatial memory in aged rats

The present study was a longitudinal study of age-related changes in performance of the 5-choice serial reaction time task, a test of visual attention. Following acquisition of the task, animals were tested on two occasions on their ability to perform the 5-choice task. In Test 1 (Young: 7 months; Aged: 13–14 months) no age-related effects on baseline performance were revealed. However, increasing the attentional load of the task revealed an impairment in choice accuracy by animals of the Aged group. In Test 2 (Young: 10–11 months; Aged 23–24 months), animals of the Aged group were significantly impaired on the baseline schedule of the task compared to the Young group. The deficit in accuracy on the task could be improved in the Aged animals by decreasing the attentional load. The results of the present study suggest a deficit in attentional function as a result of the aging process, markedly similar to that observed following lesions of the basalo-cortical cholinergic system.     

Staged decline of visual processing capacity in mild cognitive impairment and Alzheimer's disease

Visual information intake was assessed with a whole-report task in patients with probable Alzheimer's disease (AD), mild cognitive impairment (MCI), and healthy elderly control subjects. Based on a theory of visual attention (TVA), four parameters were derived characterising different aspects of visual processing capacity: perceptual threshold, iconic memory, processing speed, and visual short-term memory (VSTM) storage capacity. Results indicated increased perceptual thresholds in MCI, and an additional decline in processing speed and VSTM storage capacity in AD. Cholinomimetic medication had beneficial effects on processing speed in AD patients. Perceptual thresholds were associated with disease duration, but not with cognitive measures, while the reverse was true for speed and VSTM measures. These results reveal a staged pattern of deficits affecting pre-attentive visual processing in MCI, and attentive processing in AD. It is compatible with the amyloid cascade hypothesis and suggests that impaired visual processing is a pathological feature present already at the MCI stage and might represent a distinct marker of upcoming AD independently from memory deficits.     

Electrophysiological correlates of feature analysis during visual search

Event-related brain potentials (ERPs) were recorded from normal young adults during visual search tasks in which the stimulus arrays contained either eight identical items (homogeneous arrays) or seven identical items and one deviant item (pop-out arrays). Four experiments were conducted in which different classes of stimulus arrays were designated targets and the remaining stimulus arrays were designated nontargets. In Experiments 1 and 2, both target and nontarget pop-out stimuli elicited an enhanced anterior N2 wave and a contralaterally larger posterior P1 wave, but Experiments 3 and 4 demonstrated that these components do not reflect fully automatic pop-out detection processes. In all four experiments, target pop-outs elicited enlarged anterior P2, posterior N2, occipital P3, and parietal P3 waves. The target-elicited posterior N2 wave contained a contralateral subcomponent (N2pc) that exhibited a focus over occipital cortex in maps of current source density. The overall pattern of results was consistent with guided search models in which preattentive stimulus information is used to guide attention to task-relevant stimuli.     

Deficits in visual discrimination performance following neglect-producing unilateral lateral hypothalamic lesions in the albino rat

Adult male albino rats were trained with water reinforcement on a lateralized visual discrimination task. For six rats the correct head turn—nose poke response was to the side as the discriminative stimulus (Turn Towards group). For six others the correct response was to the side opposite from the discriminative stimulus (Turn Away group). Immediately after a rat had mastered its task, a unilateral electrolytic lesion was made under ether anesthesia through a stainless steel electrode previously implanted in the lateral hypothalamus (LH). Discrimination training continued until the rat's performance returned to criterion. The lesions produced contralateral neglect as demonstrated by deficits in turning and orientation contralateral to the lesion. The lesions also disrupted all measures of discrimination performace (% correct, latency, and duration of observing response) for trials on which responses contralateral to the lesion were correct, regardless of whether the discriminative stimulus was also contralateral (Turn Towards group) or was ipsilateral (Turn Away group). These data are interpreted as showing that contralateral neglect following LH lesions is associated with a deficit in initiating instrumental responses contralateral to the lesion. They also confirm previous results from a similar task using tactile discriminative stimuli.     

Disorders of visuospatial orientation in the frontal plane in patients with visual neglect following right or left parietal lesions

Current models of spatial neglect focus on deficits in the patients’ horizontal or midsagittal plane. However, other evidence suggests that patients with temporoparietal lesions centered on the parieto-insular-vestibular cortex show disturbed spatial perception of the subjective visual vertical and oblique orientation discrimination in another spatial plane, the frontal plane. As the relationship between neglect and spatial orientation deficits is unclear, we examined how patients with and without visual neglect perform visuospatial tasks in the roll plane and how their performance is related to neglect. Thirteen patients with predominantly right parietal lesions and left-sided neglect, 14 control patients without neglect after right-hemispheric cerebral lesions (RBD-controls), 11 patients without neglect after left-hemispheric lesions (LBD-controls), 3 patients with right-sided neglect after left parietal lesions, and 12 normal subjects were investigated. Constant errors and difference thresholds were measured with a PC-based system when the subjects had to adjust a luminous line to their subjective visual vertical, subjective visual horizontal, and in relation to an obliquely oriented reference line. Subjects were oriented with their head and body earth-vertical while sitting in a chair in total darkness. Patients with left-sided as well as those with right-sided neglect showed a significant, in most cases contraversive, tilt of the three spatial orientations (about 5° counterclockwise in the left neglect group and 5.5°–8.5° clockwise in the right neglect group). In contrast, the two patient groups without neglect as well as the normal subjects showed nearly perfect visuospatial judgements with constant errors of less than 0.8°. Difference thresholds were significantly elevated in patients with left neglect and in two of three patients with right-sided neglect, whereas normal control subjects and both control patient groups without neglect performed indistinguishably, having thresholds of one-tenth of those of the neglect patients. Tilt of all three spatial axes was significantly related to the severity of neglect (mean r for unsigned errors, 0.74; for difference thresholds, 0.40), indicating a significant contribution to the symptomatology of left and right spatial neglect. These results indicate a close although not necessarily causal link between spatial orientation deficits in the frontal plane and hemispatial neglect in patients with left or right parietal lesions, surpassing the well-documented impairments of these patients in the horizontal plane. Received: 1 August 1997 / Accepted: 1 April 1998     

The middle range of the number line orients attention to the left side of visual space

Mental representation of numbers is believed to be spatial in nature, with small numbers occupying the left and large numbers the right side of a putative mental number line. Consistent with this, presentation of numbers from the low and high ends of the mental number line induces covert shifts of spatial attention to the left and right side of visual space, respectively. However, the effect of the presentation of the middle range (containing numbers below and above the midpoint) of the number line on visual perception has so far not been studied. Here we show in two experiments, using a line bisection task and a simple target detection task, that processing of middle-range numbers affects allocation of visuospatial attention in a similar way as processing of small numbers, with attention shifted to the left side of space. We suggest that this pattern of results arises due to “anchoring” heuristics that participants use in number processing.     

Location selection in the visual domain

According to A.H.C. Van der Heijden (1992), attentional selection of visual stimuli can be considered as location selection. Depending on the type of task, location selection can be considered to be automatic (e.g., in case of abrupt onsets), directly controlled (e.g., in case of symbolic precues), or indirectly controlled (e.g., in case of visual search). In tasks with symbolic precues a contralateral enhancement of the P1 event-related potential (ERP) component has been found, whereas in visual search tasks a contralateral enhancement of the N2 component has been found. We hypothesized that both lateralized effects reflect the influence of location selection, which differs only in its moment of activation. Linear arrays (a target among five distractors, presented at two eccentricities) requiring left or right responses, were preceded by cues indicating either the precise target location or the side of the target, or by bilateral cues that indicated all array positions. Array-evoked ERPs, corrected for cue-evoked and interactive effects, showed not only lateralized but also target location-specific effects for both components, which supports the view that, depending on the type of task, either the P1 or the N2 component reflect the influence of location selection.     

The N2pc component and its links to attention shifts and spatially selective visual processing

The N2pc component has recently become a popular tool in attention research. To investigate whether this component exclusively reflects attentional target selection or also prior stages in attentional processing (covert orienting, target-unspecific spatial attention), a spatial cuing procedure was combined with a visual search task. In some blocks, informative cues indicated the side of upcoming singleton targets that were present on most trials among uniform distractors. In other blocks, cues were spatially uninformative, and no preparatory shifts of attention were possible. The N2pc in response to targets was unaffected by this manipulation, showing that this component is not associated with attention shifts. Following informative cues, an attenuated N2pc was elicited by uniform nontarget arrays, suggesting that the N2pc may also reflect spatially specific processing of stimulus features at task-relevant locations prior to target selection.     

Human brain response to visual stimulus between lower/upper visual fields and cerebral hemispheres

We studied the human brain response to visual stimulation in which a square area was randomly presented in upper and lower visual fields (VFs). Seven normal volunteers carried out a contrast-based visual search task. Magnetic responses were detected in the bilateral parietal regions at 200–250 ms after stimulus onset. We compared the response latencies and strengths of the essential single sensor and root mean square (RMS) of the regions. The former evaluates the strength of neural activity with relatively high spatial resolution, while the latter evaluates the global neural activity. The single sensor and RMS latencies for the lower left VF were significantly longer than that for the upper left (paired t-test, P < 0.05). The strengths did not differ between the upper and lower left VFs. There was no significant difference in latency or strength between the upper right and lower right VFs. These findings suggest that only left VF has different response properties in the upper versus lower VF, and that both local and global extrastriate activities are responsible for this anisotropy.     

Sensory dominance in combinations of audio,visual and haptic stimuli

Participants presented with auditory, visual, or bi-sensory audio–visual stimuli in a speeded discrimination task, fail to respond to the auditory component of the bi-sensory trials significantly more often than they fail to respond to the visual component—a ‘visual dominance’ effect. The current study investigated further the sensory dominance phenomenon in all combinations of auditory, visual and haptic stimuli. We found a similar visual dominance effect also in bi-sensory trials of combined haptic–visual stimuli, but no bias towards either sensory modality in bi-sensory trials of haptic–auditory stimuli. When presented with tri-sensory trials of combined auditory–visual–haptic stimuli, participants made more errors of responding only to two corresponding sensory signals than errors of responding only to a single sensory modality, however, there were no biases towards either sensory modality (or sensory pairs) in the distribution of both types of errors (i.e. responding only to a single stimulus or to pairs of stimuli). These results suggest that while vision can dominate both the auditory and the haptic sensory modalities, it is limited to bi-sensory combinations in which the visual signal is combined with another single stimulus. However, in a tri-sensory combination when a visual signal is presented simultaneously with both the auditory and the haptic signals, the probability of missing two signals is much smaller than of missing only one signal and therefore the visual dominance disappears.     

Acute effects of caffeine on selective attention and visual search processes

The influence of a single dose of caffeine was evaluated in focused and divided attention conditions of a visual selective search task in which subjects had to perform controlled search processes to locate a target item. Search processes were manipulated by varying display load. A dose of 3 mg/kg body weight caffeine or lactose, dissolved in a cup of decaffeinated coffee, was administered double blindly and deceptively to overnight abstinent coffee drinkers. Behavioral measures were supplemented by event-related potentials (ERPs). Subjects reacted faster in the caffeine condition. The P3b peak latency decreased after caffeine in the low display load condition and in the focused attention condition, indicating that the effects of caffeine are dependent on the number of relevant display items, not on the total number of display items presented. Search processes, as reflected in a negative ERP deflection, were not affected by caffeine.