Normative MEG data to flash and pattern reversal stimuli

We have attempted to establish normative values of components of the magnetic evoked field to flash and pattern reversal stimuli prior to clinical use of the MEG. Full visual field, binocular evoked magnetic fields were recorded from 100 subjects 16 to 86 years of age with a single channel dc Squid (BTI) second-order gradiometer at a point 5-6cm above the inion. The majority of subjects showed a large positive component (out going magnetic field) of mean latency 115 ms (SD range 2.5 -11.8 in different decades of life) to the pattern reversal stimulus. In many subjects, this P100M was preceeded and succeeded by negative deflections (in going field). About 6% of subjects showed an inverted response i.e. a PNP wave. Waveforms to flash were more variable in shape with several positive components; the most consistent having a mean latency of 110ms (SD range 6.4-23.2). Responses to both stimuli were consistent when measured on the same subject on six different occasions (SD range 4.8 to 7.3). The data suggest that norms can be established for evoked magnetic field components, in particular for the pattern reversal P100M, which could be used in the diagnosis of neuro-opthalmological disease.

Topography of visual evoked magnetic responses to pattern shift, pattern onset and flash stimuli

Different visual stimuli may activate separate channels in the visual system and produce magnetic responses from the human bran which originate from distinct regions of the visual cortex. To test this hypothesis, we have investigated the distribution of visual evoked magnetic responses to three distinct visual stimuli over the occipital region of the scalp with a DC-SQUID second-order gradiometer in an ubshielded environment. Patterned stimuli were presented full field and to the right half field, while a flash stimulus was presented full field only, in five normal subjects. Magnetic responses were recorded from 20 to 42 positions over the occipital scalp. Topographic maps were prepared of the major positive component within the first 150ms to the three stimuli, i.e., the P100m (pattern shift), C11m (pattern onset) and P2m (flash). For the pattern shift stimulus the data suggested the source of the P100m was close to the midline with the current directed towards the medial surface. The data for the pattern onset C11m suggested a source at a similar depth but with the current directed away from the midline towards the lateral surface. The flash P2m appeared to originate closer to the surface of the occipital pole than both the patterned stimuli. Hence the pattern shift (which may represent movement), and the pattern onset C11m (representing contrast and contour) appear to originate in similar areas of brain but to represent different asepcts of cortical processing. By contrast, the flash P2m (representing luminance change) appears to originate in a distinct area of visual cortex closer to the occipital pole.

The detection of automated perimetric stimuli in ocular disease

Automated perimetry has made viable a rapid threshold examination of the visual field and has reinforced the role of perimetry in the diagnostic procedure. The aim of this study was twofold: to isolate the influence of certain extraneous factors on the sensitivity gradient, since these might limit the early detection and accurate monitoring of visual field loss and to investigate the efficacy of certain novel combinations of stimulus parameters in the detection of early visual field loss. The work was carried out with particular reference to glaucoma and to ocular hypertension. The effects of media opacities on the visual field were assessed by forward intraocular light scatter (n= 15) and were found to mask diffuse glaucomatous visual field loss and underestimate focal loss. Correction of the visual field indices for the effects of forward intraocular light scatter (n= 26) showed the focal losses to be, in reality, unaffected. Measurements of back scatter underestimated forward intraocular light scatter (n= 60) and the resultant depression of the visual field. Perimetric sensitivity improved with patient learning (n= 25) and exhibited eccentricity- and depth-dependency effects whereby improvements in sensitivity were greatest for peripheral areas of the field and for those areas which initially demonstrated the lowest sensitivity. The effects of practice were retained over several months (n= 16). Perimetric sensitivity decreased during prolonged examination due to fatigue effects (n&61 19); these demonstrated a similar eccentricity-dependency, being greatest for eccentricities beyond 30o. Mean sensitivities over the range of adaptation levels employed obeyed the Weber-Fechner law (n= 10) and, as would be expected, were independent of pupil size. No relationship was found between short-term fluctuation and adaptation level. Detection of diffuse glaucomatous visual field loss was facilitated using a size III stimulus of duration 200msec at an adaptation level of 31.5asb, compared with a size III stimulus of duration 100msec at an adaptation level of 4asb (n= 20). In a pilot study (n= 10), temporal summation was found to be higher in glaucomatous patients compared with age-matched controls, although the difference was not statistically significant.

Development of visual evoked responses to tritan,red-green and luminance stimuli in human infants

The principal aim of this work was to investigate the development of the S-cone colour-opponent pathway in human infants aged 4 weeks to 6 months. This was achieved by recording transient visual evoked responses to pattern-onset stimuli along a tritanopic confusion axis (tritan stimuli) at and around the adult isoluminant match. For comparison, visual evoked responses to red-green and luminance-modulated stimuli were recorded from the same infants at the same ages. Evoked responses were also recorded from colour-normal adults for comparison with those of the infants. The transient VEP allowed observation of response morphology as luminance differences were introduced to the chromatic stimuli. In this way, an estimate of isoluminance was possible in infants. Estimated isoluminant points for a group of six infants aged 6 to 10 weeks closely approximated the adult isoluminant match. This finding has implications for the use of photometric isoluminance in infant work, and suggests that photopic spectral sensitivity is similar in infants and adults. Abnormalities of the visual evoked responses to tritan, red-green and luminance-modulated stimuli in an infant with cystic fibrosis are reported. The results suggest abnormal function of the retino-striate visual pathway in this infant, and it is argued that these may be secondary to his illness, although data from more infants with cystic fibrosis are needed to clarify this further. A group of nine healthy infants demonstrated evoked responses to tritan stimuli by 4 to 10 weeks and to red-green stimuli by 6 to 11 weeks post-term age. Responses to luminance-modulated stimuli were present in all nine infants at the earliest age tested, namely 4 weeks post-term. The slightly earlier age of onset of evoked responses to tritan stimuli than for red-green may be explained by the relatively lower cone contrast afforded by red-green stimuli. Latency of the evoked response to both types of chromatic stimuli and to luminance-modulated stimuli decreased with age at a similar rate, suggesting that the visual pathways transmitting luminance and chromatic information mature at similar rates in young infants.

Impaired sensitivity to dynamic stimuli in poor readers of a regular orthography

The mappings from grapheme to phoneme are much less consistent in English than they are for most other languages. Therefore, the differences found between English-speaking dyslexics and controls on sensory measures of temporal processing might be related more to the irregularities of English orthography than to a general deficit affecting reading ability in all languages. However, here we show that poor readers of Norwegian, a language with a relatively regular orthography, are less sensitive than controls to dynamic visual and auditory stimuli. Consistent with results from previous studies of English-readers, detection thresholds for visual motion and auditory frequency modulation (FM) were significantly higher in 19 poor readers of Norwegian compared to 22 control readers of the same age. Over two-thirds (68.4%) of the children identified as poor readers were less sensitive than controls to either or both of the visual coherent motion or auditory 2Hz FM stimuli. © 2003 Elsevier Science (USA). All rights reserved.

Interval timing in children:effects of auditory and visual pacing stimuli and relationships with reading and attention variables

Motor timing tasks have been employed in studies of neurodevelopmental disorders such as developmental dyslexia and ADHD, where they provide an index of temporal processing ability. Investigations of these disorders have used different stimulus parameters within the motor timing tasks which are likely to affect performance measures. Here we assessed the effect of auditory and visual pacing stimuli on synchronised motor timing performance and its relationship with cognitive and behavioural predictors that are commonly used in the diagnosis of these highly prevalent developmental disorders. Twenty- one children (mean age 9.6 years) completed a finger tapping task in two stimulus conditions, together with additional psychometric measures. As anticipated, synchronisation to the beat (ISI 329 ms) was less accurate in the visually paced condition. Decomposition of timing variance indicated that this effect resulted from differences in the way that visual and auditory paced tasks are processed by central timekeeping and associated peripheral implementation systems. The ability to utilise an efficient processing strategy on the visual task correlated with both reading and sustained attention skills. Dissociations between these patterns of relationship across task modality suggest that not all timing tasks are equivalent.

A working memory bias for alcohol-related stimuli depends on drinking score

We tested 44 participants with respect to their working memory (WM) performance on alcohol-related versus neutral visual stimuli. Previously an alcohol attentional bias (AAB) had been reported using these stimuli, where the attention of frequent drinkers was automatically drawn toward alcohol-related items (e.g., beer bottle). The present study set out to provide evidence for an alcohol memory bias (AMB) that would persist over longer time-scales than the AAB. The WM task we used required memorizing 4 stimuli in their correct locations and a visual interference task was administered during a 4-sec delay interval. A subsequent probe required participants to indicate whether a stimulus was shown in the correct or incorrect location. For each participant we calculated a drinking score based on 3 items derived from the Alcohol Use Questionnaire, and we observed that higher scorers better remembered alcohol-related images compared with lower scorers, particularly when these were presented in their correct locations upon recall. This provides first evidence for an AMB. It is important to highlight that this effect persisted over a 4-sec delay period including a visual interference task that erased iconic memories and diverted attention away from the encoded items, thus the AMB cannot be reduced to the previously reported AAB. Our finding calls for further investigation of alcohol-related cognitive biases in WM, and we propose a preliminary model that may guide future research. © 2012 American Psychological Association.

Optical imaging of retina in response to grating stimuli in cats

We examined the intrinsic signals in response to grating stimuli in order to determine whether the light-evoked intrinsic signals of the retina are due to changes in the photoreceptor activities induced by the image projected on to the retina or are due to neural activities of the inner retina. The retinas of the left eye of 12 cats under general anesthesia were examined by a functional imaging fundus camera. Near infrared light was used to monitor the reflectance changes (RCs) of the retina. Vertical grating were used to stimulate the retina at 4 Hz. The spatial frequencies of the gratings were 0.05, 0.11, 0.22, 0.43, 0.86, 1.73, and 3.46 cycles/degree (cpd). Ten images were averaged and used to analyze the RCs to obtain the peak value (PV) of a two dimensional fast Fourier transfer of the RCs. The wavefront aberrations (WA) were measured with a compact wavefront aberrometer and the spatial modulation transfer function (MTF) of the eye was calculated. The retinal reflectance image had a grating pattern. The PV of the spatial sensitivity curve was highest at low spatial frequencies (0.05 and 0.11 cpd), and the sensitivity decreased steeply with an increase in the spatial frequency. RCs were not detectable at 3.46 cpd. The MTF decreased gradually with increases in the spatial frequencies and was 0.68 at 3.46 cpd. The reflectance pattern of the retinal intrinsic signal elicited by grating stimuli of different spatial frequencies was different from that of the MTF. This suggests that the intrinsic signal represents not only the response of the photoreceptors but also other neuronal or vascular changes in the retina.

Comparative study of dexamethasone and vancomycin release behavior from stimuli-sensitive microgel aqueous dispersions

Stimuli-sensitive microgels of poly(N-isopropylacrylamide-co-acrylic acid) (designated as P(NIPAAm-co-AA)) were prepared through precipitation polymerization. Their capacity to load and release different drugs under different conditions, including physiological, in a controlled manner was analyzed. Two drugs were assayed and compared: dexamethasone and vancomycin. The prepared microgel particles show good thermosensitivity. In addition, the amount of cross-linker used in the preparation of the microgels does not greatly influence the drug-release capability of P(NIPAAm-co-AA)), but the amount of drug used to load the microgels did result in bigger amounts of drug released afterwards. These results imply potential application of prepared stimuli-sensitive microgel dispersions as drug-delivery systems and tissue engineering materials.

Elevated striatal and decreased dorsolateral prefrontal cortical activity in response to emotional stimuli in euthymic bipolar disorder:no associations with psychotropic medication load

To examine abnormal patterns of frontal cortical-subcortical activity in response to emotional stimuli in euthymic individuals with bipolar disorder type I in order to identify trait-like, pathophysiologic mechanisms of the disorder. We examined potential confounding effects of total psychotropic medication load and illness variables upon neural abnormalities.

Impaired executive functioning in subclinical compulsive checking with ecologically valid stimuli in a working memory task

We previously showed that working memory (WM) performance of subclinical checkers can be affected if they are presented with irrelevant but misleading information during the retention period (Harkin and Kessler, 2009, 2011). The present study differed from our previous research in the three crucial aspects. Firstly, we employed ecologically valid stimuli in form of electrical kitchen appliances on a kitchen countertop in order to address previous criticism of our research with letters in locations as these may not have tapped into the primary concerns of checkers. Secondly, we tested whether these ecological stimuli would allow us to employ a simpler (un-blocked) design while obtaining similarly robust results. Thirdly, in Experiment 2 we improved the measure of confidence as a metacognitive variable by using a quantitative scale (0–100), which indeed revealed more robust effects that were quantitatively related to accuracy of performance. The task in the present study was to memorize four appliances, including their states (on/off), and their locations on the kitchen countertop. Memory accuracy was tested for the states of appliances in Experiment 1, and for their locations in Experiment 2. Intermediate probes were identical in both experiments and were administered during retention on 66.7% of the trials with 50% resolvable and 50% irresolvable/misleading probes. Experiment 1 revealed the efficacy of the employed stimuli by revealing a general impairment of high- compared to low checkers, which confirmed the ecological validity of our stimuli. In Experiment 2 we observed the expected, more differentiated pattern: High checkers were not generally affected in their WM performance (i.e., no general capacity issue); instead they showed a particular impairment in the misleading distractor-probe condition. Also, high checkers’ confidence ratings were indicative of a general impairment in metacognitive functioning. We discuss how specific executive dysfunction and general metacognitive impairment may affect memory traces in the short- and in the long-term.

An investigation of psychological and physiological changes apparently elicited by musical stimuli

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