Impaired Center-Surround Suppression in Patients with Alzheimer's Disease

Visual surround suppression at the neural and perceptual levels

Surround suppression was initially identified as a phenomenon at the neural level in which stimuli outside the neuron's receptive field alone cannot activate responses but can modulate neural responses to stimuli covered inside the receptive field. Subsequent studies showed that surround suppression is not only a critical property of neurons across species and brain areas but also has been found in visual perceptions. More importantly, surround suppression varies across individuals and shows significant differences between normal controls and patients with certain mental disorders. Here, we combined results from related literature and summarized the findings derived from physiological and psychophysical evidence. We first outline the basic properties of surround suppression in the visual system and perceptions. Then, we mainly summarize the differences in perceptual surround suppression among different human subjects. Our review suggests that there is no consensus regarding whether the strength of perceptual surround suppression could be used as an effective index to distinguish particular populations. Then, we summarized the similar mechanisms for surround suppression and cognitive impairments to further explore the potential clinical applications of surround suppression. A clearer understanding of the mechanisms of surround suppression in neural responses and perceptions is necessary for facilitating its clinical applications.

Top-down influence of areas 21a and 7 differently affects the surround suppression of V1 neurons in cats

Surround suppression (SS) is a phenomenon whereby a neuron's response to stimuli in its central receptive field (cRF) is suppressed by stimuli extending to its surround receptive field (sRF). Recent evidence show that top-down influence contributed to SS in the primary visual cortex (V1). However, how the top-down influence from different high-level cortical areas affects SS in V1 has not been comparatively observed. The present study applied transcranial direct current stimulation (tDCS) to modulate the neural activity in area 21a (A21a) and area 7 (A7) of cats and examined the changes in the cRF and sRF of V1 neurons. We found that anode-tDCS at A21a reduced V1 neurons' cRF size and increased their response to visual stimuli in cRF, causing an improved SS strength. By contrast, anode-tDCS at A7 increased V1 neurons' sRF size and response to stimuli in cRF, also enhancing the SS. Modeling analysis based on DoG function indicated that the increased SS of V1 neurons after anode-tDCS at A21a could be explained by a center-only mechanism, whereas the improved SS after anode-tDCS at A7 might be mediated through a combined center and surround mechanism. In conclusion, A21a and A7 may affect the SS of V1 neurons through different mechanisms.

Altered visual cortex excitability in premenstrual dysphoric disorder: Evidence from magnetoencephalographic gamma oscillations and perceptual suppression

Premenstrual dysphoric disorder (PMDD) is a psychiatric condition characterized by extreme mood shifts during the luteal phase of the menstrual cycle (MC) due to abnormal sensitivity to neurosteroids and unbalanced neural excitation/inhibition (E/I) ratio. We hypothesized that in women with PMDD in the luteal phase, these factors would alter the frequency of magnetoencephalographic visual gamma oscillations, affect modulation of their power by excitatory drive, and decrease perceptual spatial suppression. Women with PMDD and control women were examined twice-during the follicular and luteal phases of their MC. We recorded visual gamma response (GR) while modulating the excitatory drive by increasing the drift rate of the high-contrast grating (static, 'slow', 'medium', and 'fast'). Contrary to our expectations, GR frequency was not affected in women with PMDD in either phase of the MC. GR power suppression, which is normally associated with a switch from the 'optimal' for GR slow drift rate to the medium drift rate, was reduced in women with PMDD and was the only GR parameter that distinguished them from control participants specifically in the luteal phase and predicted severity of their premenstrual symptoms. Over and above the atypical luteal GR suppression, in both phases of the MC women with PMDD had abnormally strong GR facilitation caused by a switch from the 'suboptimal' static to the 'optimal' slow drift rate. Perceptual spatial suppression did not differ between the groups but decreased from the follicular to the luteal phase only in PMDD women. The atypical modulation of GR power suggests that neuronal excitability in the visual cortex is constitutively elevated in PMDD and that this E/I imbalance is further exacerbated during the luteal phase. However, the unaltered GR frequency does not support the hypothesis of inhibitory neuron dysfunction in PMDD.

Visual surround suppression in people with epilepsy correlates with attentional-executive functioning, but not with epilepsy or seizure types

PURPOSE

Following reports that an index of visual surround suppression (SI) may serve as a biomarker for an imbalance of cortical excitation and inhibition in different psychiatric and neurological disorders including epilepsy, we evaluated whether SI is associated with seizure susceptibility, seizure spread, and inhibitory effects of antiseizure medication (ASM).

METHODS

In this prospective controlled study, we examined SI with a motion discrimination task in people with genetic generalized epilepsy (GGE) and focal epilepsy with and without focal to bilateral tonic-clonic seizures. Cofactors such as GABAergic ASM, attentional-executive functioning, and depression were taken into account.

RESULTS

Data of 45 patients were included in the final analysis. Suppression index was not related to epilepsy or seizure type, GABAergic ASM treatment or mood. However, SI correlated with attentional-executive functioning (r = 0.32), which in turn was associated with ASM load (r = -0.38). Repeated task administration (N = 7) proved a high stability over a one-week interval (r_tt = 0.89).

CONCLUSIONS

Our results do not support the hypothesis that SI is a reliable biomarker for mechanisms related to inhibition of seizure spread or seizure frequency, i.e., it does not seem to reflect inhibitory capacities in epilepsy. Likewise, SI did not differentiate GGE from focal epilepsy, nor was it influenced by ASM load or mode of action. Thus, in epilepsy, no added value of including SI to routine diagnostics can be concluded.

Afferent and Efferent Visual Markers of Alzheimer's Disease: A Review and Update in Early Stage Disease

Vision, which requires extensive neural involvement, is often impaired in Alzheimer's disease (AD). Over the last few decades, accumulating evidence has shown that various visual functions and structures are compromised in Alzheimer's dementia and when measured can detect those with dementia from those with normal aging. These visual changes involve both the afferent and efferent parts of the visual system, which correspond to the sensory and eye movement aspects of vision, respectively. There are fewer, but a growing number of studies, that focus on the detection of predementia stages. Visual biomarkers that detect these stages are paramount in the development of successful disease-modifying therapies by identifying appropriate research participants and in identifying those who would receive future therapies. This review provides a summary and update on common afferent and efferent visual markers of AD with a focus on mild cognitive impairment (MCI) and preclinical disease detection. We further propose future directions in this area. Given the ease of performing visual tests, the accessibility of the eye, and advances in ocular technology, visual measures have the potential to be effective, practical, and non-invasive biomarkers of AD.

Validation of motion perception of briefly displayed images using a tablet

Motion perception of briefly displayed images has been reported to be abnormal in clinical populations afflicted with schizophrenia, major depression, autism, Alzheimer’s disease and epilepsy. These abnormalities have been measured using CRT monitors connected to a computer. Given that the use of this experimental set-up in clinical environments can be difficult, we tested whether motion perception of briefly displayed images could also be measured using a tablet. For 13 participants, we found similar estimates of motion discrimination on a tablet and a CRT. This validates a tablet to measure motion perception of briefly displayed images.

Suppression and facilitation of human neural responses

Efficient neural processing depends on regulating responses through suppression and facilitation of neural activity. Utilizing a well-known visual motion paradigm that evokes behavioral suppression and facilitation, and combining five different methodologies (behavioral psychophysics, computational modeling, functional MRI, pharmacology, and magnetic resonance spectroscopy), we provide evidence that challenges commonly held assumptions about the neural processes underlying suppression and facilitation. We show that: (1) both suppression and facilitation can emerge from a single, computational principle – divisive normalization; there is no need to invoke separate neural mechanisms, (2) neural suppression and facilitation in the motion-selective area MT mirror perception, but strong suppression also occurs in earlier visual areas, and (3) suppression is not primarily driven by GABA-mediated inhibition. Thus, while commonly used spatial suppression paradigms may provide insight into neural response magnitudes in visual areas, they should not be used to infer neural inhibition.

Acute caffeine ingestion affects surround suppression of perceived contrast

Caffeine is a widely used psychostimulant that is associated with increased acetylcholine levels in mammalian brain and acetycholinesterase antagonism. Acetylcholine, a neuromodulator, plays an important role in the processing of visual information. One key example in human vision, thought to at least partly involve cholinergic neuromodulation, is perceptual surround suppression of contrast, whereby the perceived contrast of a pattern is altered by the presence of a neighbouring pattern. Perceptual surround suppression is weaker with pharmacological administration of donepezil (a centrally-acting acetylcholine enzyme inhibitor) in healthy human observers. Here, we test whether temporarily manipulating caffeine levels (from complete washout to a controlled dose of caffeine) has a similar effect on perceptual surround suppression in 21 healthy young adults (aged 20-24 years, 11 females). Neither ingestion of a caffeine pill nor placebo altered contrast judgments when the target pattern was presented on a uniform grey background ( p=0.54). With caffeine ingestion, perceptual surround suppression strength was reduced relative to baseline (prior to pill ingestion, p=0.003) and placebo ( p=0.029), irrespective of whether the surround was oriented parallel or orthogonal to the central target. While daily habitual caffeine consumption of low-to-moderate doses (<400 mg/day, estimated from a written questionnaire) is not predictive of performance, our study indicates that acute consumption of caffeine on the day of testing influences perceptual surround suppression strength. Perceptual surround suppression is predominantly attributed to inhibitory processes involving the major cortical inhibitory neurotransmitter, gamma-aminobutyric acid. Our results point to the involvement of other neuromodulators, possibly cholinergic, in perceptual surround suppression.