Sunday, 13 January 2013

Blink reflex or optical reflex

Level 1: optical reflex & Level 3: blindsight

Optical reflex: In addition to the corneal or sensory blink reflex, blinking can be elicited by visual or auditory input; i.e. bright lights, central and peripheral stimuli and loud noises. The evolutionary purpose of this reflex is again to protect the eyes from foreign bodies and bright lights. Blinking in response to a threatening visual stimulus is known as the optical reflex. 

Neuroanatomy and neurophysiology: The optical reflex is subcortical and is controlled via a pathway that bypasses the lateral geniculate body, optic radiations and visual cortex and goes directly to the superior colliculi. This pathway then relays to several brain stem and spinal nuclei via the tectobulbar and tectospinal pathways that initiate the motor response; i.e. blinking, flexing your arms upwards to protect your eyes, flexion and lateral rotation of the head away from the stimulus and finally flexion of the trunk and lower limbs to duck under the stimulus. Most of us will have experienced this reflex, for example when an insect flies suddenly into our field of vision, or when we duck or dive to prevent being hit in the face by a tennis or football, or the reflex ducking of the head to prevent hitting an overhanging branch when walking. This neuronal pathway is responsible for reflex movements in response to visual stimuli and is the pathway that allows us to play ball sports that require very rapid movements, for example tennis, cricket, baseball etc. A cricketer or tennis player hits the ball before they become consciously aware of the ball. 


Clinical Utility: The optical reflexes can be used to test visual function in patients who are semi-concious or unconscious and indicate that the retina and brain stem are functioning. This reflex is particularly useful in young children. It is also used in patient who present with blindness; if present it indicates that the lesion is very posterior, i.e. cortical, or the patient has functional or hysterical blindness. In these situations the pupillary reflexes are also present , but not the optokinetic response or reflex that is a more complex visual reflex that relies cortical functioning to activate it. 

The optical reflex is usually tested creating  a threatening lateral visual stimulus, typically your finger or hand that is brought into the lateral visual field very rapidly. There is one caveat to the so called hand method is that it may cause a gust of air over the cornea that can stimulate the corneal reflex. To prevent this from happening it can be done from behind a glass screen. This is rarely necessary in a clinical situation. 


This science experiment shows the optical blink reflex very well!


The optical reflex is responsible for the clinical phenomenon of blindsight, i.e. patients responding to visual stimuli without being of aware of it. The majority of these patients are conciously awarr of being blind, but rarely  they may deny being blind. The latter is referred to as the Anton-Babinski syndrome

The following is a short clip from the two-part documentary "Phantoms in the Brain" in which neurologist V S Ramachandran describes how the study of patients with certain types of brain damage can give us clues about the nature of consicousness and perception. For those of you who are interested in this can see both documentaries for free online. 


"Phantoms in the Brain" V S Ramachandran 

For those of you who are interested you may find this case study of interest: 

Hamm et al. Affective blindsight: intact fear conditioning to a visual cue in a cortically blind patient. Brain. 2003 Feb;126(Pt 2):267-75.

Blindsight refers to remarkable residual visual abilities of patients with damage to the primary visual cortex (V1). Recent studies revealed that such residual abilities do not apply only to relatively simple object discriminations, but that these patients can also differentially categorize and respond to emotionally salient stimuli. The current study reports on a case of intact fear conditioning to a visual cue in a male patient with complete bilateral cortical blindness. The patient was admitted to the stroke unit of the neurological department because of complete loss of vision. Both CT and structural MRI scans confirmed lesions in both territories of the posterior cerebral artery. No visual evoked potentials could be detected confirming complete cortical blindness. During fear conditioning, a visual cue predicted the occurrence of an aversive electric shock. Acoustic startle probes were presented during and between the conditioned stimuli. Relative to the control condition, startle reflexes were substantially potentiated when elicited in the presence of the conditioned stimuli. No such potentiation was observed prior to conditioning. These data suggest that fear learning to visual cues does not require a cortical representation of the conditioned stimulus in the primary sensory cortex and that subcortical pathways are sufficient to activate the fear module in humans.


Further reading: blindsight

3 comments:

  1. Involuntary reactions to stimulus resulting with instantaneous movement are called reflexes. Reflexes are apparent from birth including blinking and sneezing. There are many types of reflexes in a healthy person. A pain receptor in the skin is an example of a sensory neuron. The sensory neuron detects the pain and sends a signal that is intercepted by a motor neuron, which results in the withdrawal reflex. Since only one interaction occurs this is called a monosynaptic reflex. More complex reflexes, known as polysnaptic reflexes, involve interneurons and may be integrated through the brainstem, cerebrum or spinal cord.

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  2. The movements we use are based on reflex patterns and rhythmic movements that babies make naturally from the womb on.Some movements involve light touch, and for younger children, many of the movements can be done read more playfully.

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  3. For best sight performance, the intensity of the red-dot light source must at least roughly match the illumination level of the target. Otherwise, if the source is too dim, the aim-point dot loses itself in the brightness of the target. click here

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