BBB tutorial FOUR:
26 May University of Paisley
Snippets from discussions
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Among the topics considered were:
Brief consideration of touch
Elements of hearing
An initial look at vision.
We also looked at the opportunities afforded by the CD-ROM. This software
is made available at the Summer School, as well as other computer programs
illustrating various aspects; in particular a suite of programs about vision,
ranging from details of the retina to visual illusions
Feature Extraction
A particular feature of both touch and vision is the concentric, antagonistic
regions of the receptive fields of some cells as a result of lateral inhibition
(in the ventrobasal nucleus of the thalamus in touch, and the retinal ganglion
cells in vision). As explained in the revision note on vision,
the only stimulus which causes a change in the cell's firing rate is when
there is partial coverage of the receptive field. The two antagonistic
areas cancel each other out, so if they are equally covered it's as if
there's no stimulus at all; only when an edge passes across will there
be a signal. Thus the important information is passed on along the sensory
pathway; this makes the system much more efficient.
An example: consider the simple image of a diamond
shape against a light background shown here.
Imagine it's represented by an array of pixels, in round figures say
it's a 1000 x 1000 array - that means you need a million items of information
to describe it.
That could be called a raster image - in contrast to a vector image
in which you describe this image in terms of where the corners of the diamond
are (each as 2 values - x and y coordinates), a value for the background
colour and a value for the colour of the diamond.
In other words, you just need ten values instead of the million!
So this is what happens when the sensory system codes for significant
features instead of a dot-by-dot description of the outside world.
Thought - it's just the same with your studying of BBB
- especially if you're new to biology. You don't want to try to learn every
single word on every page - or worse still every single letter. You just
want to come away with the gist of each section of the chapters, the key
features - on more-or-less a "need to know" basis as you work through the
TMAs, CMAs and ITQs. As you come later to check up on things as background
for later books, and then in your revision towards the end of the course,
more details will slot into place.
A revision exercise for the early chapters
of Book 3 is provided.
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