Lectures 2-3.pdf

Eye’s Axial Length Control Theories Accommodative Lag Theory (2000) ○ Under-accommodation during near work ○ Image focused behind retina at fovea ○ Relative hyperopia stimulates eye growth ○ Treat with plus at near: bifocals, PALs Peripheral Refrac...

Eye’s Axial Length Control Theories Accommodative Lag Theory (2000) ○ Under-accommodation during near work ○ Image focused behind retina at fovea ○ Relative hyperopia stimulates eye growth ○ Treat with plus at near: bifocals, PALs Peripheral Refraction Theory (2020) ○ Shorter off-axis eye length I ○ Image focused behind retina in periphery ○ Relative hyperopia stimulates eye growth ○ Treat with plus in periphery Sampling of retinal images Parvo = Fovea Magno = Periphery The level of defocus present on the image does not affect the number of microsaccades generated by the individuals Myopes produce more microsaccades than emmetropes regardless of the target frequency content -> more retinal movement The number of microsaccades varies as a function of the target used. No treatment of myopia control is capable of reverting myopia Fixational Eye movements Are small displacements of the eyeballs which ensure that vision does not fade during fixation. 3 classes ○ Tremor (the smallest) ○ Drifts ○ Microsaccades (the largest) ○ Been studied with retinal stabilization techniques  Despite the existence of fixational eye movements, perception is stable. This fact has been explained by a hypothetical microsaccadic suppression mechanism. Fixational eye movement can be modulated by environmental (illumination conditions) and cognitive (attention) factors. Our visual experience is during fixation During saccades, vision is degraded Velocity ( problem with vertical gaze centers ○ Lid lag Eyelids fails to lower as gaze is lowered -> extrapyramidal syndromes  Lid retraction in the eye opposite to a ptosis is not a disorder; an example of Hering’s law Excessive closure ○ Blepharospasm Ocular blepharospasm is due to ocular disease which irritates the cornea, such as dry eyes, uveitis, keratitis Essential blepharospasm is idiopathic Oculomotor apraxia is sometimes accompanied by excessive blinking, but not specifically in relation to initiating saccades ○ Insufficient closure Diseases affecting the facial nerve nucleus (somatic motor fibers) Diseases affecting basal ganglia (Parkinson’s) Frontal lobe lesions can affect ability to wink