Biomedical Engineering Reference
In-Depth Information
magnet bore. However, all of these endeavors will require a next
generation of fMRI hardware improvement (e.g., with parallel
imaging
(76, 77)
using multiple smaller radio frequency coils)
because of the smaller BOLD responses expected.
Though a predominantly nocturnal animal, the rodent makes
use of its visual system when lighting conditions are favorable -
e.g., for vision-based navigation
(78)
. Light enters the retina,
which contains elongated photoreceptor cells that are full of
light-sensitive photopigment molecules (or opsins). Upon inter-
acting with incident photons, photoisomerization of rod and/or
cone opsins initiates a cascade of processes that rapidly lead
to a decrease in the intracellular concentration of cyclic GMP
molecules, thereby changing the cell membrane's permeability to
cations and thus altering its potential
(79)
. This graded signal is
refined further by retinal networks before being transmitted, via
the dorsal lateral geniculate nucleus of the thalamus, onto the
primary visual cortex in the contralateral occipital lobe. A paral-
lel pathway, bypassing the thalamus, transmits visual information
to the superficial layers of the contralateral superior colliculus.
The rat retina has two distinct types of cones - most sensitive to
medium wavelengths (
4.1.2. Non-tactile: Visual
Stimulation
510 nm; i.e., green) but some receptive
to very short wavelengths (
∼
360 nm; i.e., ultraviolet) - allowing
discrimination between different colors
(80)
.
Our visual stimulation setup allowed us to vary the color
and/or intensity of light pulses of different durations. At a con-
stant pulse width of 50 ms and intensity of 20-30 lux, white (
Fig.
10.4
) and green (data not shown) light evoked well-localized
cortical and sub-cortical responses which corresponded well with
known retinotopic maps of both the V1
(81, 82)
and the SC
(83)
.
At higher stimulation frequencies, responses weakened in V1 and
strengthened in DLG. These results confirmed a previous fMRI
report that examined the effect of stimulus frequency on the cor-
tical BOLD response
(37)
where there is an apparent inverse rela-
tionship between increasing flash frequency and BOLD response
amplitude. A limitation of our method, however, is the inabil-
ity to produce moving stimuli, such as drifting gratings typically
employed in studies of the visual system to map response proper-
ties of single cells
(84)
. Since the optimal tuning properties of cells
in the rat visual cortices appear to be randomly arranged
(85)
,the
probability of recording signals from an optimally excited neuron
within an ensemble is reduced. Regardless, we were able to record
ensemble multi-unit discharges in the V1 and DLG and demon-
strated their qualitative concurrence with the fMRI data. To relate
these rat studies to results from other species where vision plays
a more important role in their livelihood
(15, 18-20)
, studies are
underway to examine the degree of temporal correlation between
LFP, MUA, and BOLD signals in V1, SC, and DLG.
∼
