transient cortical excitation at the onset of visual fixation visual recognition is brain state...

Transient Cortical Excitation at the onset of visual fixation

Visual recognition is brain state dependent

How do we see the world• Scan the environment • Saccades – movement of the eyes from point of

fixation• At each fixation point – a multitude of retinal

outputs enter the system. Question – How does the onset of visual fixation (in

the absence of stimulation) effect the ongoing neural activity of the brain?

Perisaccadic Modulation • Observed in different visual pathway areas ranging from

the LGN to the prefrontal cortex• Suppression followed by enhancement• “priming effect” – for new visual input • Occurs in the absence of visual stimulation

• Pg 10 – “ The brain is perpetually active, even in the absence of environmental and body derived stimuli…in fact a main argument put forward in this book is that most of the brains activity is generated from within and perturbation of this default pattern by external inputs often causes only a minor departure from it’s robust and internally controlled program” – Rhythms of the brain

Fixation-Amplifier Hypothesis

• Developed a hypothesis based on perisaccadic modulation

• Refelective of an underlying mechanism • Increased neural responses to information

entering the retina during the point of fixation

Prove it?

• Developed a study • Animal sat in chair with eyes fixated in a set position • Eye tracker • Complete darkness • LED light• EEG + MUA • V1• 13 experimental sessions • 3 parts to study

Part 1 – CSD and MUA

Describing Fixation quantitatively: AVREC & AVMUA

• Computed grand mean for CSD – AVREC • Computed grand mean for MUA- AVMUA

Mechanisms that bring about excitation

2 possibilities:

1)The effect is local excitation triggered by some internal input – efferent copy of eye muscle command.

2) Phase resetting/phase modulation – which is the reorganization of ongoing activity without the addition of energy to the system

….

• Pg 111 – “ Brain dynamics constantly shift from the complex to the predictable…neural activity shuttles between the interference prone complexity and robust predictable oscillatory synchrony”. – Rhythms of the brain

Assessing Phase Concentration

Continue…

• These findings support the idea that oscillatory phase concentration reflects modulation of the local neural ensemble in preparation for the arrival of visual inputs generated at fixation.

Phase Histogram

2 questions still remain?

• 1)is the phase concentration purely indicative of phase resetting?

• 2) Hilbert Transform which was used to calculate the phase ignores frequency, so how can you interpret these results relative to the frequency bands seen in the EEG?

Amplitude of pre/post fixation in 3 bands:

….• Theta/delta (3-8hz) brings about phase

concentrated synchrony which is needed for optimal responding in the presence of visual fixation.

• Thus – figures 1 and 2 revealed and increase in CSD and MUA following fixation onset and figures 3 and 4 demonstrate that these increases are associated with oscillatory phase concentration and increases in spectral power.

• Do these effects reflect an increase in local cortical excitability?

Ideal vs worst phase

• Wanted to know whether or not the phase distribution during the time of fixation is associated with enhanced excitability?

• Calculated the mean CSD and MUA amplitudes

• Ideal vs worst phase.

Graphic representation of ideal & worst phase

results

• Amplitude is greater in post stimulus• Ideal phase does not reflect excitability rather

onset of fixation does.• Ideal phase is not fundamentally different

from the oscillatory phase

Explanation.

• The idea is that the fixation-related phase resetting of the oscillation can place a new retinal image in an “ideal” optimal excitability phase so that the response is amplified relative to inputs that are NOT synchronized to fixation.

• Makes use of the large amount of energy found in the oscillations

Summary:• 1) Neural modulation at the onset of fixation

reflects an underlying nonvisual mechanism that produces local neural excitation at the point of fixation in V1.

• 2) Fixation onset is associated with significant oscillatory phase concentration

• 3) Synchrony occurred in the “delta/theta” band • 4)Fixation effects reflect the ability of the brain

gaze control system to “prime” or “prepare” the visual system for temporal patterns of visual input.

Implications

• 1) enhances perception • 2) color• 3) Attention• 4) Natural Vision * Refer to last note