FIRST AMENDMENT TO

                        ASSIGNMENT AND SECURITY AGREEMENT

         THIS  FIRST  AMENDMENT  TO  ASSIGNMENT  AND  SECURITY  AGREEMENT  (this
"Agreement")  is made and entered into as of the 1st day of April,  2000, by and
between Prime Medical Operating,  Inc., a Delaware corporation  ("PMOI"),  Prime
RVC,  Inc.,  a Delaware  corporation  ("Prime  RVC"),  as assignee of  Prime/BDR
Acquisition,  L.L.C., a Delaware limited  liability  company ("Prime BDR") (PMOI
and Prime RVC are referred to herein, jointly and severally, as "Secured Party")
and David P. Bates and Jane A. Bates (collectively referred to as the "Debtor").
Unless otherwise  defined herein,  capitalized  terms used herein shall have the
meanings  ascribed to them in that certain  Assignment  and Security  Agreement,
dated effective as of September 1, 1999, among PMOI and Prime RVC as assignee of
Prime BDR (the "Assignment and Security Agreement").

                             Preliminary Statements

         Debtor and Secured Party have executed and delivered that certain Stock
Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Stock  Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation (the "Existing Center").

         Debtor  pledged  the  collateral  to  secure  certain  obligations  and
liabilities  that Debtor may now or hereafter have to Secured Party,  including,
without limitation,  any indemnity  obligations arising under the Stock Purchase
Agreement.

         The  shareholders  of  the  Existing  Center,  including  Debtor,  have
determined  that it is in their best  interests  that they,  together with Prime
RVC,  organize  and own  Horizon  Vision  Centers,  L.L.C.,  a Delaware  limited
liability  company (the "New  Center")  for the purpose of operating  new vision
centers  that will  provide  refractive  surgery  and  related  services  in the
Northern and Central California area.

         In connection  with the formation of the New Center,  Debtor has agreed
to  pledge  all of its  ownership  interest  in the  New  Center  as  additional
collateral  pursuant to the terms and  conditions of the Assignment and Security
Agreement.

                             Statement of Agreement

         NOW,  THEREFORE,  in consideration of the premises and mutual covenants
contained  herein and for other good,  valuable and binding  consideration,  the
receipt and  sufficiency of which are hereby  acknowledged,  the parties hereto,
intending to be legally bound hereby, agree as follows:


<PAGE>


         Section 1.        Amendments, Additions and Deletions to Agreement.
                           ------------------------------------------------

     a. The parties to this  Agreement  hereby agree to amend Section  1.1(a) of
the Assignment and Security Agreement to read in its entirety as follows:

                  (a) Shares. From and after the date of this Agreement, (i) all
         shares of the common stock of Horizon  Vision  Centers,  Inc., a Nevada
         corporation,  and all membership  interests of Horizon Vision  Centers,
         L.L.C., a Delaware limited liability company,  owned or acquired in any
         manner by Debtor (collectively,  the "Shares"),  (ii) any replacements,
         substitutions,  or  exchanges  of  the  certificates  representing  the
         Shares, and (iii) any and all options,  rescission rights, registration
         rights,   conversion  rights,   subscription  rights,   contractual  or
         quasi-contractual  rights,  warrants,   redemption  rights,  redemption
         proceeds,  calls,  preemptive  rights and all other rights and benefits
         pertaining to the Shares;

         Section  2.  Effect  on  Existing   Agreements.   This   Agreement   is
incorporated into the Assignment and Security Agreement by reference. Other than
as provided in this  Agreement,  the Assignment  and Security  Agreement has not
been  modified or amended and is in full force and effect.  Equity Holder hereby
affirms that it remains a party to the  Assignment  and Security  Agreement  (as
amended by this Agreement) after the execution of this Agreement. This Agreement
may be executed in a number of identical  counterparts  which,  taken  together,
shall constitute collectively one and the same agreement.

                            [Signature page follows]


<PAGE>


S-1

043838.0009  AUSTIN 189916 v1

                                SIGNATURE PAGE TO

                                 FIRST AMENDMENT

                                       TO

                        ASSIGNMENT AND SECURITY AGREEMENT

         IN  WITNESS  WHEREOF,  the  parties  hereto  have  duly  executed  this
Amendment as of the day and year first above written.

DEBTOR:


                                                David P. Bates, III



                                                Jane A. Bates


SECURED PARTY:                                  PRIME MEDICAL OPERATING, INC.


                                            By:_________________________________
                                               Teena Belcik, Treasurer


                                                PRIME/BDR ACQUISITION, L.L.C.

                                            By:_________________________________
                                               Teena Belcik, Treasurer


                                                PRIME RVC, INC.

                                            By:_________________________________
                                               Teena Belcik, Treasurer